Illumination apparatus with a catadioptric lens array that reflects and transmits light from an array of LEDs with a smaller light distribution cone

ABSTRACT

A directional illumination apparatus comprises a catadioptric micro-optic array comprising a reflective surface comprising light reflecting facets and an output transmissive surface comprising refractive structures. An array of micro-LEDs is arranged between the reflective surface and output transmissive surface and arranged to illuminate the reflective surface. The light reflecting facets and refractive structures cooperate to provide a uniform output illumination across the output aperture of the array with collimated output. A thin and efficient illumination apparatus may be used for switching display backlighting or environmental illumination applications.

TECHNICAL FIELD

The present disclosure relates to an illumination apparatus comprising aplurality of addressable light-emitting elements aligned to a pluralityof catadioptric optical elements. Such an apparatus may be used as ahigh definition directional backlight for a liquid crystal display(LCD). The apparatus may further be used to provide directional lightingfrom a spatially uniform area.

BACKGROUND

Thin substrate and polymer substrate LCD panels can provide mechanicalcharacteristics such as flexibility that is similar to organic LED(OLED) displays. Such thin substrate LCDs desirably use backlights withsimilar mechanical characteristics.

High dynamic range LCDs (HDR-LCD) can achieve dynamic ranges that aresuperior to that which can be provided by an LCD optical mode alone. Anarray of light sources such as LEDs (light emitting diodes) that isaddressed with lower resolution image data is provided in a localdimming LCD backlight, such that dark areas of an image are illuminatedby the backlight with low luminance, and bright areas are illuminatedwith high luminance.

One type of LCD backlight comprises a light guide plate, and array ofinput light sources such as LEDs at one end of the light guide plate.Light that propagates by total internal reflection within the waveguideis output by means of surface features that adjust the propagation angleof light within the waveguide and allow extraction at angles close tograzing the outside of the waveguide. Such light is directed in a normaldirection to the LCD by means of a turning film and/or rear reflectors.Such optical stacks may have high efficiency, but have multiple opticalcomponents with total backlight thickness typically 1 mm or greater.Such an edge illuminated light guide plate is not typically appropriatefor two-dimensional local dimming for HDR-LCD illumination, or free-formshaped LCD.

Other known backlights incorporate an array of light emitting diodes(LEDs) in a matrix behind the LCD such as described in US patentapplication number US20170261179 comprises a plurality of spatiallyseparated packaged LEDs and a multiple “batwing” optical elements, eachbatwing optical element arranged to direct light from the packaged LEDin a lateral direction. Such light is strongly diffused to provideoutput illumination. Such backlights require expensive pick-and-placeLED and individual optics alignment and have a high thickness andreduced efficiency in comparison to edge illuminated backlights.

Illumination systems for environmental lighting such as automobileheadlights, architectural, commercial or domestic lighting may provide anarrow directional light output distribution, for example by means offocussing optics to provide spotlighting effects, or can achieve a widedirectional light output distribution for example by means of diffusingoptics.

In this specification LED refers to an unpackaged LED die chip extracteddirectly from a monolithic wafer, i.e. a semiconductor element. This isdifferent from packaged LEDs which have been attached to a lead frame inorder to provide electrodes and may be assembled into a plastic packageto facilitate subsequent assembly. Packaged LEDs are typically ofdimension greater than 1 mm, and more typically of dimension greaterthan 3 mm and are assembled by conventional Printed Circuit Boardassembly techniques including pick and place methods. The accuracy ofcomponents placed by such assembly machines may typically be about plusor minus 30 micrometres. Such sizes and tolerances prevent applicationto very high resolution displays.

Micro-LEDs may be formed by array extraction methods in which multipleLEDs are removed from a monolithic wafer in parallel and may be arrangedwith positional tolerances that are less than 5 micrometres.

White LED lighting sources can be comprised of separate spectral bandssuch as red, green, blue and yellow, each created by a separate LEDelement. Such sources enable users to resolve the separate colours, andas a result of the separation of the sources in the lamp, can createcoloured illumination patches. It would be desirable if the sources werehomogenized so that their separation was less than the visual resolutionlimit.

Catadioptric elements combine refractive surfaces (dioptrics) andreflective surfaces (catoptrics), which may provide total internalreflection or reflection from metallised surfaces. Backlights employingcatadioptric optical elements with small output luminous intensity solidangles are described in WO2010038025 incorporated by reference herein inits entirety.

BRIEF SUMMARY

According to a first aspect of the present disclosure there is providedan illumination apparatus comprising: a plurality of LEDs arranged in anLED array, wherein the plurality of LEDs are micro-LEDs or mini-LEDs,each of the plurality of LEDs being arranged to output light having arespective light output distribution; and a plurality of catadioptricoptical elements arranged in a catadioptric optical array, eachcatadioptric optical element comprising a reflective surface and atransmissive surface facing the reflective surface, wherein: for eachcatadioptric optical element, the reflective surface is arranged toreceive light output from one or more of the LEDs through thetransmissive surface and to reflect the received light back through thetransmissive surface, thereby to provide re-directed light having arespective light output distribution, wherein the light outputdistribution of the re-directed light provided by each catadioptricoptical element has a luminous intensity half maximum solid angle thatis smaller than the luminous intensity half maximum solid angle of thelight output distribution of the light output by each of the pluralityof LEDs.

Advantageously a thin directional illumination apparatus may be providedwhich has a pitch that is significantly greater than the thickness. Auniform output illumination profile may be achieved, such that theillumination apparatus may be provided as a backlight for a transmissivespatial light modulator with high uniformity. For a given powerconsumption the head-on output luminance may be increased in comparisonwith Lambertian illumination. For a given head-on luminance the powerconsumption may be increased.

At least some of the light from the plurality of LEDs may be guided, atleast in part via total internal reflection, within the catadioptricoptical array.

Each of the plurality of LEDs may be arranged on a first surface of atleast one transmissive LED support substrate; and a transmissive outputsurface is provided by a second surface of the transmissive LED supportsubstrate; wherein the second surface of the transmissive LED supportsubstrate faces the first surface of the transmissive LED supportsubstrate. Advantageously the number of components in the illuminationapparatus may be reduced, reducing cost and complexity as well asreducing thickness.

The reflective surface of each catadioptric optical element may bearranged on a first surface of an input substrate, and a second surfaceof the input substrate facing the reflective surface comprises atransmissive input surface; wherein the first surface of thetransmissive LED support substrate faces the transmissive input surface.Advantageously device thickness may be reduced for a given opticalpathlength.

The light from the plurality of LEDs that may be guided within thecatadioptric optical array is guided, at least in part via totalinternal reflection, between the reflective surface and the transmissiveinput surface. Advantageously light from the micro-LEDs may bedistributed over large areas. The area illuminated by each micro-LED maybe increased so that fewer micro-LEDs are needed, reducing cost.

Each catadioptric optical element may comprise an optical axis.

Each optical axis may be aligned in correspondence with a respective oneor more of the LEDs, and each of the LEDs may be aligned incorrespondence with the optical axis of only one of the catadioptricoptical elements.

The illumination apparatus may further comprise a further plurality ofLEDs arranged in an LED array, wherein the further plurality of LEDs aremicro-LEDs or mini-LEDs. Each optical axis may be offset from one ormore of the LEDs of the further plurality of LEDs. Each of the LEDs ofthe further plurality of LEDs may be offset from the optical axis of atleast one of the catadioptric optical elements.

For each catadioptric optical element, the reflective surface may bearranged to receive light output from one or more of the furtherplurality of LEDs through the transmissive surface and to reflect thereceived light back through the transmissive surface, thereby to providere-directed light having a respective light output distribution.

The light output distribution of the re-directed light provided by eachcatadioptric optical element using light output from the furtherplurality of LEDs may have a luminous intensity half maximum solid anglethat is greater than the luminous intensity half maximum solid angle ofthe re-directed light provided by each catadioptric optical elementusing light output from the plurality of LEDs.

The light output distribution may be switched between a narrow outputsolid angle and a wide solid angle output. Advantageously a display maybe provided that in one mode of operation provides a low stray lightoutput for example to provide privacy, high efficiency for head-onviewing, or night operation and a second mode of operation with a widerange of viewing freedom and increased uniformity.

For each catadioptric optical element of the catadioptric optical array,the transmissive surface may comprise at least one refractive lightoutput structure arranged on the transmissive surface and aligned incorrespondence with the optical axis of the catadioptric opticalelement. Advantageously light output may be achieved with controllableangular distribution in areas of the output surface that would otherwisebe shielded by the micro-LED, providing increased uniformity for a widerange of output angles, and minimising dark spot appearance in theregion of the micro-LED. Uniformity is increased.

The input substrate may be formed as an integrated body that extendsbetween the optical axes of the plurality of catadioptric opticalelements. Advantageously a common alignment step may be provided formultiple catadioptric optical elements to the plurality of micro-LEDs,reducing cost and complexity and increasing robustness.

The LED support substrate may be formed as an integrated body thatextends between the optical axes of the plurality of catadioptricoptical elements. Advantageously many micro-LEDs may be arranged on thesubstrate in parallel, providing a known separation. Uniformity ofalignment to the catadioptric optical array may be provided, increasinguniformity, reducing alignment cost and increasing robustness.

A transparent material may be provided between the first surface of thetransmissive LED support substrate and the transmissive surface of thecatadioptric element. The light from the plurality of LEDs that isguided within the catadioptric optical array may be guided between thereflective surface and the second surface of the transmissive LEDsupport substrate. A transparent material with a lower refractive indexthan a material from which the input substrate is made may be arrangedbetween the plurality of LEDs and the transmissive surfaces of thecatadioptric optical elements. The transparent material may be air.Advantageously robustness may be increased and sensitivity to thermalvariations reduced. Further the spatial uniformity of light output maybe increased.

The reflective surface of the catadioptric optical array may comprise areflective layer formed on the reflective surface. The reflective layermay comprise a metal material. The reflective layer may extend to coverthe reflective surface of the catadioptric optical array. Advantageouslylight that is incident below the critical angle at the reflectivesurface may be reflected for output in directions that are near to thenormal direction or in directions that guide within the catadioptricoptical array. The overall efficiency of reflection from the reflectedsurface may be increased, increasing output efficiency.

The reflective surface of each catadioptric optical element may comprisea plurality of light reflecting facets.

For each catadioptric optical element the transmissive surface of theinput substrate may further comprise a refractive light input structurealigned in correspondence with a respective optical axis of thecatadioptric optical element. Each refractive light input structure maybe arranged between the transmissive input surface and the reflectivesurface of the input substrate. The luminous intensity of light that isdirected for output at locations near to the optical axis may beincreased. Advantageously the appearance of dark spots near to themicro-LED may be reduced and uniformity increased.

In at least one catadioptric cross-sectional plane through its opticalaxis the refractive light input structure may comprise a plurality ofpairs of oppositely inclined refractive input facets.

In at least one catadioptric cross-sectional plane the plurality ofpairs of inclined input facets may be inclined at equal and oppositeinclination angles. In the plane of the catadioptric optical array theplurality of pairs of inclined refractive input facets may be circularlyor elliptically symmetric. Advantageously the thickness of the outputmicrostructure may be reduced, reducing total thickness.

The transmissive surface of the input substrate may comprise planarregions between the refractive light input structures. The reflectivesurface may comprise reflective planar regions between at least some ofthe light reflecting facets of the reflective surface. Advantageouslylight may be guided to outer regions, increasing device area, reducingthickness and reducing total cost of micro-LEDs.

The reflective surface of each catadioptric optical element may comprisea reflective light input structure that is arranged between thereflective surface and the transmissive input surface of the inputsubstrate. In at least one catadioptric cross-sectional plane throughits optical axis the reflective light input structure may comprises afirst inner surface and a second inner surface facing the first innersurface. For each catadioptric optical element of the catadioptricoptical array, the refractive light input structure and reflective lightinput structure may be arranged to direct at least some light from therespective aligned at least one LED to be the light that is guidedwithin the catadioptric optical array. In at least one catadioptriccross-sectional plane through its optical axis the first and secondinner surfaces may comprise curved reflective surfaces. In at least onecatadioptric cross-sectional plane through its optical axis the lightreflecting facets of the reflective surface may be provided by pairs ofinclined facets that are inclined with opposing inclination angles.Advantageously light may be guided to outer regions, increasing devicearea, reducing thickness and reducing total cost of micro-LEDs.

Some of the light reflecting facets of the reflective surface may bearranged to direct at least some light through the transmissive outputsurface of the catadioptric optical element in a direction substantiallynormal to the transmissive output surface. Advantageously additionallight deflection films are not used, achieving reduced thickness andcomplexity of operation.

In the plane of the catadioptric array the light reflecting facets maybe circularly or elliptically symmetric about the optical axis of eachcatadioptric optical element. The plurality of light reflecting facetsof each catadioptric optical element may be concentric with the opticalaxis of said catadioptric optical element. Advantageously the lightoutput may be provided across the area of the catadioptric opticalelement with high uniformity.

In at least one catadioptric cross-sectional plane through its opticalaxis the light reflecting facets of a catadioptric optical element maybe arranged with a separation that decreases with distance from theoptical axis of the catadioptric element. For each catadioptric opticalelement the length of the light reflecting facets may increase withdistance from the optical axis of the respective catadioptric opticalelement. For each catadioptric optical element the total area of thelight reflecting facets may increase with the distance from the opticalaxis of the respective catadioptric optical element. For eachcatadioptric optical element, the total area of the at least one lightreflecting facet at a distance, r from the optical axis may beproportional to the distance, r. Advantageously the luminance providedfrom each region of the catadioptric optical element may besubstantially the same, independent of distance from the micro-LED. Muraand Moiré effects may be minimised in a backlight apparatus.

Some of the light reflecting facets arranged on the reflective surfaceof the catadioptric optical element may be arranged to direct light thathas not guided within the catadioptric optical array. Advantageouslysome of the light near to the optical axis may be output to provideluminance that is substantially as the regions in which light that hasbeen guided is output.

The illumination apparatus may comprise a plurality of opaque maskregions wherein the first surface of the transmissive LED supportsubstrate for each catadioptric optical element comprises an opaque maskregion that is aligned with an optical axis of the catadioptric opticalelement. A respective one or more of the LEDs of the plurality of LEDsmay be arranged between the mask region and the reflective surface. Theopaque mask region may be provided between the refractive light outputstructure and the respective one or more of the LEDs of the plurality ofLEDs. Advantageously the output angular directional distribution near tothe optical axis may be substantially the same as the output angulardirectional distribution for regions not near to the optical axis.

The plurality of opaque mask regions may be provided by LED addressingelectrodes. Advantageously the complexity of fabrication of the LEDsupport substrate may be reduced, reducing cost.

Some light reflecting facets of the reflective surface of the respectivecatadioptric optical element may be arranged to direct light to therefractive light output structure. In at least one catadioptriccross-sectional plane through its optical axis the refractive lightoutput structure may comprise a concave refractive surface arranged toprovide negative optical power. In at least one catadioptriccross-sectional plane the refractive light output structure may comprisea plurality of pairs of oppositely inclined transmissive lightdeflecting facets. For each catadioptric optical element the pluralityof pairs of oppositely inclined transmissive light deflecting facets maybe circularly or elliptically symmetric in the plane of the transmissiveoutput surface about the optical axis of the catadioptric opticalelement. Advantageously the angular directional distribution of lightclose to the optical axis may be substantially the same as the angulardirectional distribution from other regions of the catadioptric opticalelement.

The illumination apparatus may further comprise diffuser structuresarranged on at least one surface of the transmissive LED supportsubstrate. Advantageously additional diffuser layers may be reduced oreliminated, reducing thickness.

The angular light output distribution of light from the refractive lightoutput structure may be substantially the same as the angular lightoutput distribution of light from the plurality of reflective lightreflecting facets that is transmitted through regions of thetransmissive output substrate that do not comprise a refractive lightoutput structure. Advantageously output uniformity across thecatadioptric optical element may be substantially the same for a widerange of viewing directions.

The illumination apparatus may further comprise a reflective polariserarranged to provide polarisation recirculation of light reflected fromthe reflective surface of the catadioptric optical element.Advantageously the efficiency of polarised output may be increased.Further the recirculated light may be efficiently recycled by thereflective surface without additional reflective layers, reducing costand complexity. Flexible, curved and bendable illumination structuresmay be conveniently provided by a thin illumination apparatus withreduced number of layers.

The ratio of luminous intensity half maximum solid angle of the outputlight cone to the luminous intensity half maximum solid angle of aLambertian light source may be less than 1, preferably less than 50%,more preferably less than 25% and most preferably less than 10%.Advantageously display luminance may be increased for a given powerconsumption, or display power consumption may be reduced for a givendisplay luminance. Further, a backlight for a privacy display may beprovided that has low luminance at high viewing angles.

The illumination apparatus may further comprise a wavelength conversionlayer. Advantageously white light output may be provided.

The wavelength conversion layer may be arranged between the LEDs of theplurality of LEDs and the reflective surface of each catadioptricoptical element. Advantageously the angular directional distribution ofoutput may be reduced.

The wavelength conversion layer may be arranged to receive light fromthe catadioptric optical array. Advantageously the wavelength diffusionlayer may provide a further diffuser function and provide increaseduniformity of output.

The electrodes of each of the LEDs of the plurality LEDs may berespectively connected to one column addressing electrode and one rowaddressing electrode. Advantageously the plurality of micro-LEDs mayprovide local area dimming for high dynamic range operation incooperation with an LCD. Display contrast may be increased in comparisonto area illumination.

The illumination apparatus may further comprise an integrated circuitcontrolling one or more LEDs and located within the LED array. Theintegrated circuit may comprise a storage or memory or latchingfunction. Advantageously the width of the edges of the illuminationapparatus may be reduced for small bezel width and flexibility.

The LEDs of the plurality of LEDs may be from a monolithic waferarranged in an array with their original monolithic wafer positions andorientations relative to each other preserved. In at least onedirection, for at least one pair of the plurality of LEDs in the atleast one direction, for each respective pair there may have been atleast one respective LED in the monolithic wafer that was positioned inthe monolithic wafer between the pair of LEDs in the at least onedirection and that is not positioned between them in the array of LEDs.Advantageously the pitch of the micro-LEDs may be determined at the timeof transfer from the monolithic wafer to the substrate. The catadioptricoptical element may have substantially the same pitch such that largenumbers of micro-LEDs may be precisely aligned to large numbers ofcatadioptric optical elements. Advantageously cost and complexity ofalignment of the illumination apparatus is reduced.

The LEDs of the plurality of LEDs may be micro-LEDs of width or diameterless than 300 micrometres, preferably less than 200 micrometres and morepreferably less than 100 micrometres. In the at least one catadioptriccross-sectional plane the distance between the transmissive outputsurface and reflective surface may be less than 750 micrometres,preferably less than 500 micrometres and more preferably less than 250micrometres. Advantageously, a thin and bright directional illuminationapparatus may be provided. High resolution local area dimming may befurther provided.

According to a second aspect of the present disclosure there is provideda display apparatus comprising an illumination apparatus according tothe first aspect and a transmissive spatial light modulator arranged toreceive light that has transmitted through the transmissive LED supportsubstrate. Advantageously a thin display may be provided with local areadimming, high contrast, high resolution, high uniformity, free-formshapes, very low bezel width and flexibility. Further such a display mayprovide power savings, very high luminance in brightly lit environments,low stray light in low illuminance environments and privacy operationsuch that the display is only visible from a restricted viewing angle.

According to a third aspect of the present disclosure there is provideda backlight apparatus for a liquid crystal display comprising anillumination apparatus according to the first aspect.

According to a fourth aspect of the present disclosure there is providedan illumination apparatus comprising: a plurality of LEDs, the pluralityof LEDs being arranged in an LED array, wherein the LEDs of theplurality of LEDs are micro-LEDs; and a catadioptric optical array toprovide a light output distribution, the light output distribution beingof light output from the LEDs of the plurality of LEDs; wherein: thecatadioptric optical array comprises a plurality of catadioptric opticalelements, the plurality of catadioptric optical elements being arrangedin an array, each of the catadioptric optical elements of the pluralityof catadioptric optical elements comprising an optical axis; the opticalaxis of each of the catadioptric optical elements is aligned incorrespondence with a respective one or more of the LEDs of theplurality of LEDs, each of the LEDs of the plurality of LEDs beingaligned with the optical axis of only one of the respective catadioptricoptical elements of the catadioptric optical array; each catadioptricoptical element of the catadioptric optical array comprises: areflective surface comprising a plurality of light reflecting facetsarranged on the reflective surface and aligned in correspondence withthe optical axis; and a transmissive output surface wherein thetransmissive output surface faces the reflective surface; the pluralityof LEDs is arranged between the reflective surface and the transmissiveoutput surface and the plurality of LEDs is arranged to illuminate thereflective surface; at least some of the light from the plurality ofLEDs is guided within the catadioptric optical array; and the pluralityof light reflecting facets is arranged to direct light through thetransmissive output surface of the catadioptric optical array; whereinthe light output distribution has a luminous intensity half maximumsolid angle that is smaller than the luminous intensity half maximumsolid angle of the light output distribution from each of the pluralityof LEDs.

Advantageously a thin directional illumination apparatus may be providedwhich has a pitch that is significantly greater than the thickness. Auniform output illumination profile may be achieved, such that theillumination apparatus may be provided as a backlight for a transmissivespatial light modulator with high uniformity. For a given powerconsumption the head-on output luminance may be increased in comparisonwith Lambertian illumination. For a given head-on luminance the powerconsumption may be increased.

The plurality of LEDs may be arranged on the first surface of atransmissive LED support substrate; and the transmissive output surfacemay be provided by the second surface of the transmissive LED supportsubstrate; wherein the second surface of the transmissive LED supportsubstrate faces the first surface of the transmissive LED supportsubstrate. Advantageously the number of components in the illuminationapparatus may be reduced, reducing cost and complexity as well asreducing thickness.

The reflective surface of each catadioptric optical element may bearranged on the first surface of an input substrate, and the secondsurface of the input substrate facing the reflective surface maycomprise a transmissive input surface; wherein the first surface of thetransmissive LED support substrate faces the transmissive input surface.Advantageously device thickness may be reduced for a given opticalpathlength.

The light from the plurality of LEDs that is guided within thecatadioptric optical array may be guided between the reflective surfaceand the transmissive input surface. Advantageously light from themicro-LEDs may be distributed over large areas. The area illuminated byeach micro-LED may be increased so that fewer micro-LEDs are needed,reducing cost.

For each catadioptric optical element of the catadioptric optical arraythe transmissive output surface may comprise at least one refractivelight output structure arranged on the transmissive output surface andaligned in correspondence with the optical axis of the catadioptricoptical element. Advantageously light output may be achieved withcontrollable angular distribution in areas of the output surface thatwould otherwise be shielded by the micro-LED, providing increaseduniformity for a wide range of output angles, and minimising dark spotappearance in the region of the micro-LED. Uniformity is increased.

The input substrate may be formed as an integrated body that extendsbetween the optical axes of the plurality of catadioptric opticalelements. Advantageously a common alignment step may be provided formultiple catadioptric optical elements to the plurality of micro-LEDs,reducing cost and complexity and increasing robustness.

The LED support substrate may be formed as an integrated body thatextends between the optical axes of the plurality of catadioptricoptical elements. Advantageously many micro-LEDs may be arranged on thesubstrate in parallel, providing a known separation. Uniformity ofalignment to the catadioptric optical array may be provided, increasinguniformity, reducing alignment cost and increasing robustness.

A transparent material may be provided between the first surface of thetransmissive LED support substrate and the transmissive input surface;and the light from the plurality of LEDs that is guided within thecatadioptric optical array may be guided between the reflective surfaceand the second surface of the transmissive LED support substrate.Advantageously robustness may be increased and sensitivity to thermalvariations reduced.

The reflective surface of the catadioptric optical array may comprise areflective layer formed on the reflective surface. The reflective layermay extend to cover the reflective surface of the catadioptric opticalarray. Advantageously light that is incident below the critical angle atthe reflective surface may be reflected for output in directions thatare near to the normal direction or in directions that guide within thecatadioptric optical array. The overall efficiency of reflection fromthe reflected surface may be increased, increasing output efficiency.

For each catadioptric optical element the transmissive surface of theinput substrate may further comprises a refractive light input structurealigned to the respective optical axis; wherein each light inputstructure is arranged between the transmissive input surface and thereflective surface of the input substrate. The luminous intensity oflight that is directed for output at locations near to the optical axismay be increased. Advantageously the appearance of dark spots near tothe micro-LED may be reduced and uniformity increased.

In at least one catadioptric cross-sectional plane through its opticalaxis the refractive light input structure may comprise a plurality ofpairs of oppositely inclined refractive input facets.

In at least one catadioptric cross-sectional plane the plurality ofpairs of inclined input facets may be inclined at equal and oppositeinclination angles; and in the plane of the catadioptric optical arraythe plurality of pairs of inclined refractive input facets may becircularly or elliptically symmetric. Advantageously the thickness ofthe output microstructure may be reduced, reducing total thickness.

The transmissive surface of the input substrate may comprise planarregions between the refractive light input structures. The reflectivesurface may comprise reflective planar regions between at least some ofthe light reflecting facets of the reflective surface. Advantageouslylight may be guided to outer regions, increasing device area, reducingthickness and reducing total cost of micro-LEDs.

The reflective surface of each catadioptric optical element may comprisea reflective light input structure that may be arranged between thereflective surface and the transmissive input surface of the inputsubstrate; wherein in at least one catadioptric cross-sectional planethrough its optical axis the reflective light input structure maycomprise a first inner surface and a second inner surface facing thefirst inner surface; wherein for each catadioptric optical element ofthe catadioptric optical array, the refractive light input structure andreflective light input structure may be arranged to direct at least somelight from the respective aligned at least one LED to be the light thatis guided within the catadioptric optical array. In at least onecatadioptric cross-sectional plane through its optical axis the firstand second inner surfaces may comprise curved reflective surfaces. In atleast one catadioptric cross-sectional plane through its optical axisthe light reflecting facets of the reflective surface may be provided bypairs of inclined facets that are inclined with opposing inclinationangles. Advantageously light may be guided to outer regions, increasingdevice area, reducing thickness and reducing total cost of micro-LEDs.

Some of the light reflecting facets of the reflective surface may bearranged to direct at least some light through the transmissive outputsurface of the catadioptric optical element in a direction substantiallynormal to the transmissive output surface. Advantageously additionallight deflection films are not used, achieving reduced thickness andcomplexity of operation.

In the plane of the catadioptric array the light reflecting facets maybe circularly or elliptically symmetric about the optical axis of eachcatadioptric optical element. The plurality of light reflecting facetsof each of the catadioptric optical elements may be concentric with theoptical axis of said catadioptric optical element. Advantageously thelight output may be provided across the area of the catadioptric opticalelement with high uniformity.

For each catadioptric optical element the length of the light reflectingfacets may increase with distance from the optical axis of therespective catadioptric optical element. For each catadioptric opticalelement the total area of the light reflecting facets may increase withthe distance from the optical axis of the respective catadioptricoptical element. For each catadioptric optical element, the total areaof the at least one light reflecting facet at a distance, r from theoptical axis may be proportional to the distance, r. In at least onecatadioptric cross-sectional plane through its optical axis the lightreflecting facets of a catadioptric optical element may be arranged witha separation that decreases with distance from the optical axis of thecatadioptric element. In the plane of a catadioptric optical element thelength of the light reflecting facets may increase with distance fromthe optical axis of the respective catadioptric optical element. In theplane of a catadioptric optical element the total area of the lightreflecting facets may increase with the distance from the optical axisof the respective catadioptric optical element. In the plane of acatadioptric optical element the total area of the light reflectingfacets may be proportional to the distance from the optical axis of therespective catadioptric optical element. Advantageously the luminanceprovided from each region of the catadioptric optical element may besubstantially the same, independent of distance from the micro-LED. Muraand Moiré effects may be minimised in a backlight apparatus.

Some of the light reflecting facets arranged on the reflective surfaceof the catadioptric optical element may be arranged to direct light thathas not guided within the catadioptric optical array. Advantageouslysome of the light near to the optical axis may be output to provideluminance that is substantially as the regions in which light that hasbeen guided is output.

The illumination apparatus may comprise a plurality of opaque maskregions wherein the first surface of the transmissive LED supportsubstrate for each catadioptric optical element may comprise an opaquemask region that is aligned with an optical axis of the catadioptricoptical element; wherein a respective one or more of the LEDs of theplurality of LEDs may be arranged between the mask region and thereflective surface; and wherein the opaque mask region may be providedbetween the refractive light output structure and the respective one ormore of the LEDs of the plurality of LEDs. Advantageously the outputangular directional distribution near to the optical axis may besubstantially the same as the output angular directional distributionfor regions not near to the optical axis.

The plurality of opaque mask regions may be provided by LED addressingelectrodes. Advantageously the complexity of fabrication of the LEDsupport substrate may be reduced, reducing cost.

Some light reflecting facets of the reflective surface of the respectivecatadioptric optical element may be arranged to direct light to therefractive light output structure. In at least one catadioptriccross-sectional plane through its optical axis the refractive lightoutput structure may comprise a concave refractive surface arranged toprovide negative optical power. In at least one catadioptriccross-sectional plane the refractive light output structure may comprisea plurality of pairs of oppositely inclined transmissive lightdeflecting facets. For each catadioptric optical element the pluralityof pairs of oppositely inclined transmissive light deflecting facets maybe circularly or elliptically symmetric in the plane of the transmissiveoutput surface about the optical axis of the catadioptric opticalelement.

Advantageously the angular directional distribution of light close tothe optical axis may be substantially the same as the angulardirectional distribution from other regions of the catadioptric opticalelement.

The illumination apparatus may further comprise diffuser structuresarranged on at least one surface of the transmissive LED supportsubstrate. Advantageously additional diffuser layers may be reduced oreliminated, reducing thickness.

The angular light output distribution of light from the refractive lightoutput structure may be substantially the same as the angular lightoutput distribution of light from the plurality of reflective lightreflecting facets that is transmitted through regions of thetransmissive output substrate that do not comprise a refractive lightoutput structure. Advantageously output uniformity across thecatadioptric optical element may be substantially the same for a widerange of viewing directions.

The illumination apparatus may further comprise a reflective polariserarranged to provide polarisation recirculation of light reflected fromthe reflective surface of the catadioptric optical element.Advantageously the efficiency of polarised output may be increased.Further the recirculated light may be efficiently recycled by thereflective surface without additional reflective layers, reducing costand complexity. Flexible, curved and bendable illumination structuresmay be conveniently provided by a thin illumination apparatus withreduced number of layers.

The ratio of luminous intensity half maximum solid angle of the outputlight cone to the luminous intensity half maximum solid angle of aLambertian light source may be less than 1, preferably less than 50%,more preferably less than 25% and most preferably less than 10%.Advantageously display luminance may be increased for a given powerconsumption, or display power consumption may be reduced for a givendisplay luminance. Further, a backlight for a privacy display may beprovided that has low luminance at high viewing angles.

The illumination apparatus may further comprise a wavelength conversionlayer. Advantageously white light output may be provided.

The wavelength conversion layer may be arranged between the LEDs of theplurality of LEDs and the reflective surface of each catadioptricoptical element. Advantageously the angular directional distribution ofoutput may be reduced.

The wavelength conversion layer may be arranged to receive light fromcatadioptric optical array. Advantageously the wavelength diffusionlayer may provide a further diffuser function and provide increaseduniformity of output.

The electrodes of each of the LEDs of the plurality LEDs may berespectively connected to one column addressing electrode and one rowaddressing electrode. Advantageously the plurality of micro-LEDs mayprovide local area dimming for high dynamic range operation incooperation with an LCD. Display contrast may be increased in comparisonto area illumination.

The illumination apparatus may further comprise an integrated circuitcontrolling one or more LEDs and located within the LED array. Theintegrated circuit may comprise a memory or latching function.Advantageously the width of the edges of the illumination apparatus maybe reduced for small bezel width and flexibility.

The LEDs of the plurality of LEDs may be from a monolithic waferarranged in an array with their original monolithic wafer positions andorientations relative to each other preserved; and wherein in at leastone direction, for at least one pair of the plurality of LEDs in the atleast one direction, for each respective pair there was at least onerespective LED in the monolithic wafer that was positioned in themonolithic wafer between the pair of LEDs in the at least one directionand that is not positioned between them in the array of LEDs.Advantageously the pitch of the micro-LEDs may be determined at the timeof transfer from the monolithic wafer to the substrate. The catadioptricoptical element may have substantially the same pitch such that largenumbers of micro-LEDs may be precisely aligned to large numbers ofcatadioptric optical elements. Advantageously cost and complexity ofalignment of the illumination apparatus is reduced.

The LEDs of the plurality of LEDs may be micro-LEDs of width or diameterless than 300 micrometres, preferably less than 200 micrometres and morepreferably less than 100 micrometres. In the at least one catadioptriccross-sectional plane the distance between the transmissive outputsurface and reflective surface may be less than 750 micrometres,preferably less than 500 micrometres and more preferably less than 250micrometres. Advantageously, a thin and bright directional illuminationapparatus may be provided. High resolution local area dimming may befurther provided.

According to a fifth aspect of the present disclosure there is provideda display apparatus comprising the illumination apparatus of the fourthaspect and a transmissive spatial light modulator arranged to receivelight that has transmitted through the transmissive LED supportsubstrate. Advantageously a thin display may be provided with local areadimming, high contrast, high resolution, high uniformity, free-formshapes, very low bezel width and flexibility. Further such a display mayprovide power savings, very high luminance in brightly lit environments,low stray light in low illuminance environments and privacy operationsuch that the display is only visible from a restricted viewing angle.

According to a sixth aspect of the present disclosure there is providedan illumination apparatus comprising: a plurality of LEDs, the pluralityof LEDs being arranged in an LED array, wherein the LEDs of theplurality of LEDs are mini-LEDs; a transmissive LED support substratecomprising a first surface and a second surface facing the first surfacewherein the plurality of LEDs is arranged on the first surface of thetransmissive LED support substrate; and a catadioptric optical array toprovide a light output distribution, the light output distribution beingof light output from the LEDs of the plurality of LEDs; wherein: thecatadioptric optical array comprises a plurality of catadioptric opticalelements, the plurality of catadioptric optical elements being arrangedin an array, each of the catadioptric optical elements of the pluralityof catadioptric optical elements comprising an optical axis; the opticalaxis of each of the catadioptric optical elements is aligned incorrespondence with a respective one or more of the LEDs of theplurality of LEDs, each of the LEDs of the plurality of LEDs beingaligned with the optical axis of only one of the respective catadioptricoptical elements of the catadioptric optical array; the catadioptricoptical array comprises a reflective surface and a transmissive surfacefacing the reflective surface; the first surface of the transmissive LEDsupport substrate faces the transmissive surface of the catadioptricoptical array; at least some of the light from the plurality of LEDs isguided within the catadioptric optical array between the reflectivesurface and the transmissive surface; and each catadioptric opticalelement of the catadioptric optical array comprises a plurality of lightreflecting facets arranged on the reflective surface; wherein at leastsome of the plurality of light reflecting facets are arranged to directlight that is guided between the reflective surface and the transmissivesurface of the catadioptric optical array through the transmissivesurface of the catadioptric optical array and through the transmissiveLED support substrate. The catadioptric optical array may be formed asan integrated body and the reflective surface and the transmissivesurface of the catadioptric optical array extend between the pluralityof LEDs. The reflective surface of the catadioptric optical array maycomprise a reflective coating that extends to cover the reflectivesurface of the catadioptric optical array. The transmissive surface ofeach catadioptric optical element may comprise a refractive light inputstructure that is arranged between the transmissive surface and thereflective surface; wherein each refractive light input structure may bealigned with the optical axis of the catadioptric optical element;wherein in at least one catadioptric cross-sectional plane through itsoptical axis the refractive light input structure may comprise aplurality of pairs of oppositely inclined refractive facets that areinclined at equal and opposite inclination angles; and in the plane ofthe catadioptric array the plurality of pairs of inclined refractivefacets may be circularly or elliptically symmetric. The reflectivesurface of each catadioptric optical element may comprise a reflectivelight input structure that is arranged between the reflective surfaceand the transmissive input surface of the input substrate to the lightguiding surface; wherein in at least one catadioptric cross-sectionalplane through its optical axis the reflective input structure comprisesa first inner surface and a second inner surface facing the first innersurface; and the refractive light input structure and reflective lightinput structure may be arranged to direct light from the respectivealigned at least one LED to be guided within the catadioptric opticalarray between the reflective surface and the transmissive surface of thecatadioptric optical array. The light reflecting facets of thereflective surface may be provided by pairs of inclined facets that areinclined with opposing inclination angles; and wherein in the plane ofthe catadioptric array the light reflecting facets may be circularly orelliptically symmetric and are concentric with the optical axis of saidcatadioptric optical element. The light reflecting facets of thereflective surface may be arranged to direct at least some light throughthe transmissive surface of the catadioptric optical array and thetransmissive LED substrate in a direction normal to the surface of thetransmissive LED substrate. In at least one catadioptric cross-sectionalplane through its optical axis the light reflecting facets of acatadioptric optical element may be arranged with a separation thatdecreases with distance from the optical axis of the catadioptricelement. In the plane of the catadioptric array the length of the lightreflecting facets may increase with distance from the optical axis ofthe respective catadioptric optical element; wherein the total area ofthe light reflecting facets increases with the distance from the opticalaxis of the respective catadioptric optical element. The illuminationapparatus may further comprise light reflecting facets arranged on thereflective surface of the catadioptric optical element and arranged todirect light from an aligned LED through the transmissive LED substratethat has not guided between the reflective and transmissive surfaces ofthe catadioptric optical element. The first surface of the transmissiveLED support substrate may comprise a plurality of opaque mask regionsthat are aligned with the optical axis of the catadioptric opticalelement wherein a respective one or more of the LEDs of the plurality ofLEDs is arranged on each of the opaque mask regions; wherein theplurality of opaque mask regions may be provided by LED addressingelectrodes. Some light reflecting facets of the reflective surface ofthe respective catadioptric optical element may be arranged to directlight through the first surface of the LED support substrate to theoutput refractive structures arranged at the second surface of thetransmissive LED support substrate and aligned with the optical axis ofthe catadioptric optical element; wherein in at least one catadioptriccross-sectional plane the output refractive structure may comprise aplurality of pairs of oppositely inclined transmissive light deflectingfacets. The illumination apparatus may further comprise diffuserstructures arranged on at least one surface of the transmissive LEDsupport substrate.

According to a seventh aspect of the present disclosure there isprovided a backlight apparatus for a liquid crystal display comprisingthe illumination apparatus of the sixth aspect.

Such an apparatus may be used for LCD backlighting or for domestic orprofessional lighting.

These and other features and advantages of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingfigures, wherein like reference numbers indicate similar parts.

FIG. 1 is a schematic diagram illustrating in side perspective view adisplay apparatus comprising a backlight comprising a micro-LED and acatadioptric optical element arranged to illuminate an LCD;

FIG. 2 is a schematic diagram illustrating in top view a displayapparatus comprising a backlight comprising a micro-LED array and acatadioptric optical element array arranged to illuminate an LCD;

FIG. 3 is a schematic diagram illustrating in expanded front view layersof a display apparatus comprising a backlight comprising a micro-LEDarray and a catadioptric optical element array arranged to illuminate anLCD;

FIG. 4A is a schematic diagram illustrating in top view light rays froma micro-LED and catadioptric optical element comprising a light inputmicrostructure for a catadioptric input substrate;

FIG. 4B is a schematic diagram illustrating in top view the transmissiveoutput surface of the LED support substrate arranged to provide diffusedoutput;

FIG. 4C is a schematic graph illustrating the variation of outputluminous intensity from a refractive microstructure of FIG. 4A on thetransmissive side of the input substrate of a catadioptric opticalelement;

FIG. 5A is a schematic graph illustrating the luminous intensityvariation of a directional distribution in comparison to a Lambertiandirectional distribution;

FIG. 5B is a schematic graph illustrating the luminous intensityvariation of a normalised directional distribution in comparison to aLambertian directional distribution;

FIG. 5C is a schematic graph illustrating the variation of simulatedoutput luminous intensity against output angles for the arrangement ofFIG. 4A;

FIG. 6 is a schematic graph illustrating the variation of simulatedoutput luminous intensity against position across the output aperturefor the arrangement of FIG. 4A;

FIG. 7 is a schematic diagram illustrating in top view light rays from amicro-LED and catadioptric optical element comprising a planartransmissive surface for a catadioptric input substrate;

FIG. 8 is a schematic graph illustrating the variation of outputluminance from a refractive microstructure of FIG. 7 on the transmissiveside of the input substrate of a catadioptric optical element;

FIG. 9A is a schematic graph illustrating the simulated variation oftotal output luminous intensity against output angles for thearrangement of FIG. 7;

FIG. 9B is a schematic graph illustrating the simulated variation oftotal output luminous intensity against position across the outputaperture for the arrangement of FIG. 7;

FIG. 10 is a schematic diagram illustrating in front view an arrangementof a diffuser structure provided on at least one surface of thetransmissive LED support substrate;

FIG. 11A is a schematic diagram illustrating in front view anarrangement of light reflecting facets of the reflective surface with ahexagonal packing of catadioptric optical elements;

FIG. 11B is a schematic diagram illustrating in front view arrangementsof light reflecting facets of the reflective surface with a squarepacking of catadioptric optical elements;

FIG. 11C is a schematic diagram illustrating in perspective side view aone dimensional catadioptric optical element array with reflectivesurface structure of the type illustrated in FIG. 2 and alignedmicro-LED array;

FIG. 11D is a schematic diagram illustrating in side view a reflectivesurface structure for a catadioptric optical element array;

FIG. 11E is a schematic diagram illustrating in side view a catadioptricoptical element array comprising the structure of FIG. 11D;

FIG. 11F is a schematic diagram illustrating in perspective side view atwo dimensional catadioptric optical element array with reflectivesurface structure of the type illustrated in FIGS. 11D-E and alignedmicro-LED array;

FIG. 12A is a schematic diagram illustrating in front view a reflectivesurface structure for a catadioptric optical element array and first andsecond micro-LED arrays arranged to provide switching between a narrowangle and wide angle field of view;

FIG. 12B is a schematic graph illustrating the angular profile for thefirst micro-LED array of FIG. 12A;

FIG. 12C is a schematic graph illustrating the spatial uniformity acrossthe array for the first micro-LED array of FIG. 12A;

FIG. 12D is a schematic graph illustrating the angular profile for thesecond micro-LED array of FIG. 12A;

FIG. 12E is a schematic graph illustrating the spatial uniformity acrossthe array for the second micro-LED array of FIG. 12A;

FIG. 13A is a schematic diagram illustrating in top view light the inputregion of a catadioptric optical element and aligned micro-LED;

FIG. 13B is a schematic diagram illustrating in top view light rays froma micro-LED and catadioptric optical element comprising an LED supportsubstrate that is attached to the catadioptric substrate;

FIG. 13C is a schematic diagram illustrating in top view light rays froma micro-LED and catadioptric optical element comprising a transparentLED support substrate wherein the second side of the transparentsubstrate is attached to the catadioptric substrate;

FIG. 14A is a schematic diagram illustrating in top view a catadioptricoptical array and plurality of micro-LEDs wherein the reflective inputstructures do not comprise an integrated body;

FIG. 14B is a schematic diagram illustrating in top view a curveddisplay comprising a catadioptric backlight and LCD;

FIG. 15A is a schematic diagram illustrating in side perspective view adisplay apparatus comprising a backlight comprising a micro-LED and acatadioptric optical element arranged to illuminate an LCD wherein thecatadioptric optical element does not comprise regions arranged toprovide guiding with the catadioptric optical element;

FIG. 15B is a schematic diagram illustrating in side view a catadioptricoptical element arranged wherein the catadioptric optical element doesnot comprise regions arranged to provide guiding with the catadioptricoptical element and the micro-LEDs are arranged in alignment with theoptical axes;

FIG. 15C is a schematic diagram illustrating in side perspective view atwo dimensional catadioptric optical element and aligned array ofmicro-LEDs;

FIG. 15D is a schematic diagram illustrating in side perspective view aone dimensional catadioptric optical element and aligned array ofmicro-LEDs;

FIG. 15E is a schematic diagram illustrating in side view a catadioptricoptical element arranged wherein the micro-LEDs are arranged offset fromthe optical axes of respective aligned catadioptric optical element;

FIG. 15F is a schematic diagram illustrating in side view a catadioptricoptical element arranged wherein the catadioptric optical elementwherein first and second micro-LEDs are arranged offset from the opticalaxes of respective aligned catadioptric optical element by first andsecond distances respectively;

FIG. 15G is a schematic diagram illustrating in side view a catadioptricoptical element similar to FIG. 15B wherein the reflective surface isprovided by a Fresnel reflector;

FIGS. 16A-D are schematic diagrams illustrating addressing systems forthe plurality of LEDs;

FIG. 17A is a schematic diagram illustrating in side perspective view atool for forming a plurality of refractive light input structures;

FIG. 17B is a schematic diagram illustrating in side perspective view aninput substrate comprising a plurality of refractive light inputstructures;

FIG. 17C is a schematic diagram illustrating in side perspective view atool for forming a plurality of reflective light input structures;

FIG. 17D is a schematic diagram illustrating in side perspective view aninput substrate comprising a plurality of refractive light inputstructures and a reflective surface comprising a plurality reflectivestructures;

FIG. 17E is a schematic diagram illustrating in side perspective view acoated input substrate;

FIG. 18A is a schematic diagram illustrating in side perspective view anLED support substrate comprising a plurality of refractive light outputstructures and a diffusing surface;

FIG. 18B is a schematic diagram illustrating in side perspective view anLED support substrate further comprising a plurality of addressingelectrodes;

FIG. 18C is a schematic diagram illustrating in side perspective view anLED support substrate further comprising a plurality of opaque maskregions;

FIG. 19A is a schematic diagram illustrating in side perspective view amonolithic LED wafer;

FIG. 19B is a schematic diagram illustrating in side perspective viewextraction of a sparse array of micro-LEDs from a monolithic LED wafer;

FIG. 19C is a schematic diagram illustrating in side perspective viewplacement of the sparse array of micro-LEDs from a monolithic LED waferof FIG. 19A onto the LED support substrate of FIG. 18C; and

FIG. 20 is a schematic diagram illustrating in side perspective viewassembly of a backlight comprising an input substrate and LED supportsubstrate, in accordance with the present disclosure.

DETAILED DESCRIPTION

It would be desirable to provide a thin illumination apparatus fordisplay, display backlighting or for domestic or professionalenvironmental lighting. Environmental lighting may include illuminationof a room, office, building, scene, street, equipment, or otherillumination environment. Display backlighting means an illuminationapparatus arranged to illuminate a transmissive spatial light modulatorsuch as a liquid crystal display. The micro-LEDs of a display backlightmay be provided with image information, for example in high dynamicrange operation as will be described herein. However, in general pixeldata is provided by the spatial light modulator.

It would further be desirable to provide a thin backlight for a spatiallight modulator that can provide local area dimming for high dynamicrange, a thin package, a widely spaced array of light sources and highuniformity. It would be further desirable to provide thin, flexible andfree-form shapes (for example circular) backlights for thin substrateLCDs with very low bezel widths that achieve appropriate light outputdistributions with high uniformity, high efficiency and HDR capability.

The structure and operation of various switchable display devices willnow be described. In this description, common elements have commonreference numerals. It is noted that the disclosure relating to anyelement applies to each device in which the same or correspondingelement is provided. Accordingly, for brevity such disclosure is notrepeated.

FIG. 1 is a schematic diagram illustrating in side perspective view adisplay apparatus comprising a backlight comprising a micro-LED and aunit cell of a catadioptric optical element arranged to illuminate anLCD 200; and FIG. 2 is a schematic diagram illustrating in top view in across sectional plane through its optical axes 11 a,11 b, a displayapparatus comprising a backlight comprising a micro-LED array and twounit cells 38 a, 38 b of a catadioptric optical element array 100arranged to illuminate an LCD 200.

In operation, micro-LEDs 3 provide light rays in a direction that isaway from a spatial light modulator 48 and towards a reflective surface64 as indicated by arrow 103. Light rays are reflected at reflectivesurface 64 and directed back through the catadioptric optical element 38as indicated by arrow 105. In the present embodiments, the foldedoptical path illustrated by arrows 103, 105 advantageously achieves highoptical efficiency, low thickness and high uniformity over areas thatare much greater than the area of the individual micro-LEDs, as will bedescribed further herein.

An illumination apparatus comprises a plurality of LEDs, the pluralityof LEDs being arranged in an LED array, wherein the LEDs of theplurality of LEDs are micro-LEDs 3; and a catadioptric optical array 100to provide a light output distribution, the light output distributionbeing of light output from the LEDs of the plurality of micro-LEDs 3.

The catadioptric optical array 100 comprises a plurality of catadioptricoptical elements 38, the plurality of catadioptric optical elements 38being arranged in an array, each of the catadioptric optical elements 38of the plurality of catadioptric optical elements comprising an opticalaxis 11, thus in FIG. 2, adjacent optical axes 11 a, 11 b are associatedwith each catadioptric optical element 38 a, 38 b respectively, eachcatadioptric optical element 38 a, 38 b being a unit cell of array 100.The optical axis 11 of each of the catadioptric optical elements 38 isaligned in correspondence with a respective one or more of themicro-LEDs 3 of the plurality of micro-LEDs 3, each of the micro-LEDs 3of the plurality of micro-LEDs 3 being aligned with the optical axis 11of only one of the respective catadioptric optical elements 38 of thecatadioptric optical array 100.

The plurality of catadioptric optical elements 38 may typically bearranged as a two-dimensional array in the plane of the catadioptricoptical array 100. Alternatively the catadioptric optical array may beone dimensional, that is elongate in a direction in the plane of thecatadioptric optical array.

Each catadioptric optical element 38 of the catadioptric optical array100 comprises a reflective surface 64 comprising a plurality of lightreflecting facets 70, 72 arranged on the reflective surface 64 andaligned with the optical axis 11. Each catadioptric optical element 38further comprises a transmissive output surface 52 comprising at leastone refractive light output structure 56 arranged on the transmissiveoutput surface 52 and aligned with respect to the optical axis 11. Thetransmissive output surface 52 faces the reflective surface 64.

A display apparatus comprises the backlight apparatus comprisingmicro-LED 3 array and catadioptric optical array 100, and a transmissivespatial light modulator 200 arranged to receive light that hastransmitted through the transmissive LED support substrate 50. Typicallythe transmissive spatial light modulator 200 comprises a liquid crystaldisplay with input polariser 204, substrate 206, liquid crystal layer208, substrate 210 and output polariser 212. Further layers comprisingreflective polariser 202 and diffuser 203 may be provided.

Advantageously addressable illumination can be provided in a thinoptical stack. Substrates 206, 210 may comprise thin substrates, such as150 micrometres thickness or less that may be flexible. Thin substratesmay be micro-sheet glass, glass that has been thinned bychemical-mechanical polishing, or polymer substrates such as polyimideor colourless polyimide. Advantageously an LCD that may be curved orused for flexible display may be provided as will be described furtherhereinbelow.

Further the total thickness of the spatial light modulator 200 may beless than 1 mm, preferably less than 500 micrometres, and mostpreferably less than 250 micrometres for applications such as mobiledisplay. Control electronics may be provided within the active area ofthe spatial light modulator to provide substantially zero bezel, forexample bezel widths of less than 500 micrometres. Further free-formshapes for the spatial light modulator, such as circular display may beachieved as will be described further hereinbelow.

It would be desirable to provide a backlight optical system that has thesame or less thickness than the spatial light modulator 48, is flexibleand can provide illumination of very low bezel width wherein the x-axisand y-axis dimensions of the display are similar, and free-formdisplays. Further it would be desirable to provide an addressable arrayof light sources to illuminate the spatial light modulator 200 toachieve high dynamic range, advantageously increasing image contrast.

For the purposes of the present disclosure, the plurality of LEDs aremicro-LEDs 3 of width or diameter less than 300 micrometres, preferablyless than 200 micrometres and more preferably less than 100 micrometres.LEDs that have minimum width or diameter between 100 and 500 micrometresmay also be referred to as mini-LEDs.

Such micro-LEDs 3 have a minimum width or diameter that may besubstantially larger than the width of red, green and blue image pixels220, 222, 224 provided on the spatial light modulator 200

In an illustrative example, the pixels 220, 222, 224 may have a pitch of25×75 micrometres for example. Micro-LED 3 may have a width or diameterthat is 100 micrometres, and catadioptric optical element 38 may have apitch in at least one catadioptric cross section that is 1 mm. Thusmicro-LED 3 may be arranged to illuminate more than 500 image pixels220, 222, 224.

The plurality of micro-LEDs 3 is arranged on the first surface 54 of atransmissive LED support substrate 50 and the transmissive outputsurface 52 of the catadioptric optical element 38 is provided by thesecond surface of the transmissive LED support substrate 50. The secondsurface 52 of the transmissive LED support substrate 50 faces the firstsurface 54 of the transmissive LED support substrate 50. The LED supportsubstrate 50 is formed as an integrated body that extends between theoptical axes 11 of the plurality of catadioptric optical elements 38.

Advantageously during manufacture and assembly the plurality ofmicro-LEDs 3 may be conveniently assembled on surface 54 of thetransmissive LED support substrate 50, that may comprise electrodes 7,8and other electronic components as will be described furtherhereinbelow.

Electrodes 8 are arranged to provide electrical connection to themicro-LED 3 and are provided with signals from backlight controller 130.Display controller 230 is arranged to provide image pixels 220, 222, 224with image data and may further provide backlight controller 130 withimage data such that the LEDs 3 of the LED array are provided with imagedata. High dynamic range operation may be provided to advantageouslyachieve increased image contrast.

The reflective surface 64 of each catadioptric optical element 38 isarranged on the first surface 62 of an input substrate 60, and atransmissive input surface 62 faces the reflective surface 64. The firstsurface 54 of the transmissive LED support substrate 50 faces thetransmissive input surface 62.

The input substrate 60 is formed as an integrated body that extendsbetween the optical axes 11 of the plurality of catadioptric opticalelements 38. Advantageously during manufacture and assembly opticalstructures may be arranged on the substrate 60 such that a large areabacklight may be conveniently provided. Further, alignment with thetransmissive LED support substrate 50 may be conveniently provided overa large area.

The reflective surface 64 of the catadioptric optical array 100comprises a reflective layer 65 formed on the reflective surface 64. Thereflective layer 65 extends to cover the reflective surface 64 of thecatadioptric optical array 100. The reflective layer may be provided forexample by a metal layer that may be formed on the surface 64 by meansof evaporation, sputtering, spray or dip coating. Suitable metalsinclude silver or aluminium that may be provided with protective layersto minimise corrosion and provide barrier layers to water and oxygeningress.

The metal reflective layer 65 achieves efficient reflection of light forangles of incidence below the critical angle at a surface if the surfacewere uncoated. In conventional edge illuminated waveguides for LCDbacklights, metals undesirably provide substantial losses because oflarge number of surface reflections that take place during guiding alongthe waveguide. In the present embodiments, the number of reflectionsfrom the metal layer is small in comparison to conventional waveguidesand thus losses from metal layers 65 are substantially reduced.Advantageously a thin catadioptric optical element 38 can be providedwith high efficiency with micro-LEDs that are arranged within the activearea of the spatial light modulator 48 and do not provide hot-spots ofillumination around said micro-LEDs 3.

The metal layer 65 may alternatively be patterned, for example to coverthe region of reflective light input structure 68. Advantageously lossesdue to reflections at metal layers 65 may be reduced.

Adhesive regions 80 may be further provided between the input substrate60 and transmissive LED support substrate 50. Adhesive regions 80 mayprovide attachment of the two layers to advantageously achieve robustalignment and reduced sensitivity to thermal changes.

In other words, an illumination apparatus may comprise a plurality ofLEDs, the plurality of LEDs being arranged in an LED array, wherein theLEDs of the plurality of LEDs are micro-LEDs 3. The mini-LEDs 3 may bearranged on a transmissive LED support substrate 50 comprising a firstsurface 54 and a second surface 52 facing the first surface 54 whereinthe plurality of mini-LEDs 3 is arranged on the first surface 54 of thetransmissive LED support substrate 50. Further a catadioptric opticalarray 100 may be provided to provide a light output distribution, thelight output distribution being of light output from the mini-LEDs 3 ofthe plurality of mini-LEDs 3. The catadioptric optical array 100comprises a plurality of catadioptric optical elements 38, the pluralityof catadioptric optical elements 38 being arranged in an array, each ofthe catadioptric optical elements 38 of the plurality of catadioptricoptical elements 38 comprising an optical axis 11. The optical axis 11of each of the catadioptric optical elements 38 is aligned incorrespondence with a respective one or more of the mini-LEDs 3 of theplurality of mini-LEDs 3, each of the mini-LEDs 3 of the plurality ofmini-LEDs 3 being aligned with the optical axis 11 of only one of therespective catadioptric optical elements 38 of the catadioptric opticalarray 100. The catadioptric optical array 100 comprises a reflectivesurface 64 and a transmissive surface facing the reflective surface 64.The first surface of the transmissive LED support substrate 50 faces thetransmissive surface of the catadioptric optical array 100. At leastsome of the light from the plurality of mini-LEDs 3 is guided within thecatadioptric optical array 100 between the reflective surface 64 and thetransmissive surface. Each catadioptric optical element 38 of thecatadioptric optical array 100 comprises a plurality of light reflectingfacets 70 arranged on the reflective surface 64; wherein at least someof the plurality of light reflecting facets 70 are arranged to directlight that is guided between the reflective surface 64 and thetransmissive surface of the catadioptric optical array 100 through thetransmissive surface of the catadioptric optical array 100 and throughthe transmissive LED support substrate 50.

The arrangement of optical structures in the plane of catadioptricoptical array 100 will now be described.

FIG. 3 is a schematic diagram illustrating in expanded front view layersof a display apparatus comprising a backlight comprising a micro-LEDarray and a catadioptric optical element array 100 arranged toilluminate an LCD 200.

Reflective surface 64 comprises a plurality of tessellated polygons, inthis illustration hexagonal regions 90 are centred on optical axes 11 a,11 b and 11 c. Hexagonal regions are arranged over the width of thebacklight and represent the location of each catadioptric opticalelement 38 in the plane of the reflective surface 64. Each catadioptricoptical element 38 comprises reflective light input structure 68 andreflective facets 70, 72, 74 as will be described further hereinbelow.

Transmissive input surface 62 comprises hexagonal regions 91 centred onthe same respective optical axes 11 a, 11 b and 11 c as for thereflective surface 64. Refractive light input structures 66 are arrangedin alignment with optical axes 11 a, 11 b and 11 c.

The plurality of micro-LEDs 3 is centred on hexagonal regions 92 and onthe same respective optical axes 11 a, 11 b and 11 c as for thereflective surface 64.

The first surface 54 of the transmissive LED support substrate 50comprises hexagonal regions 93 centred on the same respective opticalaxes 11 a, 11 b and 11 c as for the reflective surface 64, as well asopaque regions 7 that may be electrodes, as well as addressingelectrodes 8 to provide electrical connectivity to each of themicro-LEDs 3, each arranged in alignment with optical axes 11 a, 11 band 11 c.

The transmissive output surface 52 comprises hexagonal regions 94centred on the same respective optical axes 11 a, 11 b and 11 c as forthe reflective surface 64, as well as refractive light output structure56.

Light output through the transmissive output surface 52 may be incidenton a diffuser 203, reflective polariser and spatial light modulator 200comprising input polariser 204, liquid crystal pixel layer 208 andoutput polariser 212. For illustrative purposes the location of thehexagonal structures in alignment with the spatial light modulator 200is shown, illustrating that many pixels may be illuminated by eachcatadioptric optical element 38. The arrangement of catadioptric opticalelements 38 in the catadioptric optical array may be provided tominimise appearance of mura in the final output image. Further thearrangement may be adjusted to optimise the appearance of high dynamicrange addressing of the plurality of micro-LEDs 3.

It would be desirable for the light from the plurality of micro-LEDs 3to be distributed such that the output luminance is substantiallyspatially uniform over the area of each catadioptric optical element 38,and the luminous intensity directional distribution is substantially thesame for each region over the area. Further it would be desirable toprovide such spatially and directionally uniform distribution of lightoutput across adjacent catadioptric optical elements 38 of thecatadioptric optical array 100 to achieve desirable uniform illuminationof the spatial light modulator 200.

Features of the arrangement of FIG. 3 not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

The operation of the catadioptric optical array 100 that achievesspatially uniform distribution of light output will now be furtherdescribed with reference to certain raypaths of light from the micro-LED3.

FIG. 4A is a schematic diagram illustrating in top view light rays froma micro-LED 3 and catadioptric optical element 38 comprising arefractive light input microstructure 66 for a catadioptric inputsubstrate 60.

The plurality of micro-LEDs 3 is arranged between the reflective surface64 and the transmissive output surface 52. The first surface 54 of thetransmissive LED support substrate 50 for each catadioptric opticalelement 38 comprises an opaque mask region 7 wherein a respective one ormore of the micro-LEDs 3 of the plurality of micro-LEDs 3 is arrangedbetween the mask region 7 and the reflective surface 64.

The opaque mask region 7 is further provided between the refractivelight output structure 56 and the respective one or more of themicro-LEDs 3 of the plurality of micro-LEDs 3. Opaque mask region 7 isaligned with an optical axis 11 of the catadioptric optical element 38and may be provided by an addressing electrode of the micro-LED 3 asillustrated in FIG. 1 for example comprising aluminium or otherreflective metal materials. Drive electrodes 8 that are not in theopaque mask region 7 may be provided by transparent conductor materialssuch as ITO or silver nanowires to advantageously achieve increasedefficiency of light transmission through the transparent LED supportsubstrate 50.

The plurality of micro-LEDs 3 is arranged to illuminate the reflectivesurface 64 with light rays 300, 302, 304, 306, 308. Light rays 300, 302,304, 306, 308 from micro-LED 3 are incident on wavelength conversionlayer 5 aligned to the micro-LED 3. The micro-LED may comprise a blueemitting gallium nitride LED chip and the wavelength conversion layer 5may for example comprise phosphor or quantum dot materials that may bearranged to convert some of the blue light into yellow light or red andgreen light. Alternatively, the micro-LED 3 may comprise an ultra-violetemitting LED and the wavelength conversion material is arranged toprovide white light output.

Light rays 300, 302, 304, 306, 308 are directed towards the reflectivesurface 64 and prevented from illuminating the transmissive outputsubstrate 52 directly by opaque mask regions 7 that shield thetransmissive output surface 52 from light from the micro-LED 3. By wayof comparison with the present embodiments, if opaque mask regions 7were not present, light rays from the micro-LED 3 would be transmitteddirectly to the transmissive output surface 52 and be output from thesurface 52 with a Lambertian luminous intensity directional distributionthat would undesirably provide a hot spot at the LED location forcertain viewing angles. Advantageously the opaque mask regions 7 achievereduced appearance of hot spots.

The opaque mask regions 7 may further be reflective such that light rayspropagating with the catadioptric optical array that are reflected fromthe reflective surface 64 towards the micro-LED 3 are reflected andrecirculated. Advantageously backlight efficiency may be increased.

Light rays 300 illustrate a raypath from the micro-LED 3 that passesthrough refractive light input structure 66. Light input structure 66provides a redistribution of luminous intensity angular distributionfrom the micro-LED and will be described further below. Light ray 300 isincident onto reflective surface 64 at reflective light input structure68 that extends from the reflective surface 64 to the transmissiveoutput surface 52. In at least one catadioptric cross-sectional planethrough its optical axis 11 the reflective light input structure 68comprises a first inner surface 69 a and a second inner surface 69 bfacing the first inner surface. The first and second inner surfaces 69a, 69 b may comprise curved reflective surfaces 69 a, 69 b.Advantageously light may be efficiently reflected within the inputsubstrate 60.

For each catadioptric optical element 38 of the catadioptric opticalarray 100, the refractive light input structure 66 and reflective lightinput structure 68 are arranged to direct at least some light from therespective aligned at least one micro-LED 3 to be the light ray 300 thatis guided within the catadioptric optical array 100. Light rays 300 arereflected by the surface 69 a within the input substrate 60 and arefurther incident on transmissive input substrate 62 that comprisesplanar regions 63 that extend between the input structures 66. Ray 300has an angle of incidence greater than the critical angle at theinterface of the input substrate 60 to the gap 99 that may comprise airand is guided within the catadioptric optical element 38 between thereflective surface 64 and transmissive input surface 62 such that it isdirected back towards reflective surface 64 where it is incident ontoinclined facet 70 a.

Advantageously light ray 300 may be directed to regions of thecatadioptric optical element 38 that are remote from the micro-LED 3.Further the guiding of light ray 300 within the input substrate 60achieves a reduction in the total thickness 75 of the catadioptricoptical array 100.

The plurality of light reflecting facets 70 is arranged to direct lightthrough the transmissive output surface 52 of the catadioptric opticalarray 100. Some of the light reflecting facets 70 of the reflectivesurface 64 are arranged to direct at least some light through thetransmissive output surface 52 of the catadioptric optical element 38 ina direction substantially normal to the transmissive output surface 52.In other words, facet 70 a may be inclined to deflect guided light ray300 in a direction that is substantially parallel to the optical axis11. Other light rays (not shown) that guide within the input substrate60 may be provided at other output angles that are close to thedirection of the optical axis 11, as will be described further below.

The light reflecting facets 70 are illuminated by light cones from thelight input structure 68 that has a limited cone angle of illumination.The angular output from the facets 70 when output into air thus has anon-Lambertian output. The facets 70 may further be arranged as elementsof a curved surface to achieve increased collimation across the width ofthe element 38. The cone angle of illumination from the catadioptricoptical element may be non-Lambertian as will be described below.Advantageously display efficiency may be increased for head on viewingin comparison to Lambertian backlights. Further for displays in whichangular output similar to Lambertian displays is desirable, such as forhighly curved displays, uniform illumination of a Lambertian diffusercan be achieved. Further a backlight for a privacy display may beprovided with reduced off-axis luminance such that the display is notclearly visible for off-axis viewing locations.

Light ray 302 illustrates a raypath that after reflection from curvedinner surface 69 b is incident on reflective planar regions 71 betweenat least some of the light reflecting facets 70 of the reflectivesurface 64. Light ray 300 is guided within the input substrate 60 suchthat it is directed into a neighbouring catadioptric optical element 38of the catadioptric optical array 100.

Such a ray from a neighbouring catadioptric optical element 38 isfurther illustrated by ray 306. The light reflecting facets 70 of thereflective surface 64 are provided by pairs of inclined facets 70 a, 70b that are inclined with opposing inclination angles. Light rays 306 areincident on inclined reflective facet 70 b to be directed to the outputsurface 52 in a direction that is substantially normal to the plane ofthe substrates 60, 50.

Advantageously light rays 302, 306 may provide some mixing betweenneighbouring catadioptric optical elements 38. Such mixing may provide aspatial uniformity at the nominal interface between the two elements 38.Further, the luminous intensity directional distributions aresubstantially the same at the nominal interface, achieving improveduniformity for a wide range of viewing angles. Advantageously displayuniformity is improved.

It would be desirable to achieve uniform output luminous intensitydistribution near to the optical axis 11, illustrated by light ray 304for light that has not guided within the input substrate 60. Some of thelight reflecting facets 72 arranged on the reflective surface 64 of thecatadioptric optical element 38 are arranged to direct light ray 304that has not guided within the catadioptric optical array 100.Advantageously spatial uniformity may be increased while achievingluminous intensity angular directional distribution that is the sameacross different regions of the catadioptric optical element 38.

Light rays 308 may be provided from the region of the transmissiveoutput surface 52 between the opaque mask 7 and the spatial lightmodulator 200. Light ray 308 illustrates a raypath that achievesillumination in an otherwise shadowed region of surface 52. Lightreflecting facets 74 of the reflective surface 64 of the respectivecatadioptric optical element 38 are arranged to direct light to therefractive light output structure 56. In at least one catadioptriccross-sectional plane the refractive light output structure 56 comprisesa plurality of pairs of oppositely inclined transmissive lightdeflecting facets 57 a, 57 b. As illustrated in FIG. 3, the refractivelight output structure 56 comprising plurality of pairs of oppositelyinclined transmissive light deflecting facets 57 a, 57 b may becircularly or elliptically symmetric, in the plane of the transmissiveoutput surface 52. Hexagonal boundaries may be used in arrays ofcatadioptric optical elements to provide continuous arrays. The facets70, 72, 74 may be concentric with the optical axis, but may beinterrupted in the outer regions of each catadioptric optical element38.

To continue the illustrative embodiment, the facets 57 a, 57 b may beplanar facets with a surface normal direction that has an inclination of60 degrees to the optical axis 11 in at least one catadioptric crosssectional profile.

Adhesive regions 80 may further comprise a transparent material so thatsome of the light rays 316 that guide in the input substrate 60 aredirected to guide within the transparent LED support substrate 50. Suchguiding light may provide reduction of non-uniformities and may beextracted by means of diffusion on or in the substrate 50 or byrefractive light output structure 56.

The operation of the light input structure 66 will now be describedfurther.

As illustrated in FIG. 4A, for each catadioptric optical element 38 inat least one catadioptric cross-sectional plane the transmissive surface62 of the input substrate 60 comprises a refractive light inputstructure 66 aligned to the respective optical axis 11. Each light inputstructure 66 extends from the transmissive input surface 62 to thereflective surface 64 of the input substrate 60. In other words, therefractive light input structure 66 may be a microstructure that isarranged on the transmissive input surface 62 of the input substrate 60.The refractive input structure 66 may be fully or partially recessedinto the substrate 60 or may be proud of the surface 62. The refractiveinput structure 66 may be formed on the surface of a transparentsubstrate such as a polymer or glass, for example by means of UVcasting, printing, embossing or injection moulding.

The refractive light input structure 66 comprises a plurality of pairsof oppositely inclined refractive input facets 67 a, 67 b that may beinclined at equal and opposite inclination angles to the normaldirection in the at least one catadioptric cross-sectional plane. Asillustrated in FIG. 3, in the plane of the catadioptric optical array 38the plurality of pairs of inclined refractive input facets 67 a, 67 bare circularly or elliptically symmetric. In an illustrative embodiment,each input facet may have a planar surface and the angle of each surface67 a, 67 b to the optical axis 11 may be 52 degrees. The pitch of themicrostructure may be 50 microns for example. Advantageously refractiveinput facets 67 a, 67 b may have a low depth, minimising total backlightthickness 75.

It would be desirable to recycle unwanted polarised light from a spatiallight modulator 200 comprising an LCD. Reflective polariser 202 isarranged to provide polarisation recirculation of light reflected fromthe reflective surface 64 of the catadioptric optical element 38.Incident light rays 308, 310 are typically unpolarised and a singlepolarisation state 311 is transmitted, while an orthogonal polarisationstate is reflected. Optional retarder 201 that may be a quarterwaveplate may be arranged to modify the reflected polarisation state tothe planar regions 71 of the reflective surface 64. Reflected light hasa polarisation state that is transformed into an orthogonal polarisationstate and transmitted through the reflective polariser 202.Advantageously efficiency may be improved. Further, diffuser layersarranged on the reflective polariser and/or retarder may be arranged tofurther increase spatial uniformity and reduce mura visibility. Incomparison to conventional light recirculating backlights, thickness andcost is reduced because no separate rear reflector layer (that maytypically have a thickness of 0.1 mm or more) is used.

Features of the arrangement of FIG. 4A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

Diffuser 203 may be provided by a surface and/or bulk diffusingstructure. It may be desirable to provide diffusion for light that isoutput from the output refractive microstructure 56.

FIG. 4B is a schematic diagram illustrating in top view a detail of thecatadioptric optical element 38 comprising the refractive light inputstructure 66 and refractive light output structure 56 of thetransmissive output surface 52 of the LED support substrate arranged towherein the structure 56 further provides diffused output.

Transmissive light deflecting facets 57 a, 57 b may be provided withcurved surfaces, such that light cone solid angle 342 for light rays 308from the surfaces 57 a, 57 b is substantially the same as the cone 340from diffuser surface 352 that may be arranged on the surface 52.

Thus, the angular light output distribution of light from the refractivelight output structure 56 is substantially the same as the angular lightoutput distribution of light from the plurality of reflective lightreflecting facets 70 that is transmitted through regions of thetransmissive output substrate that do not comprise a refractive lightoutput structure 56.

Features of the arrangement of FIG. 4B not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

The output of the micro-LED 3 and refractive light input structure 66will now be further described. In FIG. 4B, light rays that are emittedin the normal direction from the micro-LED 3 are directed towards therefractive light input structure 66 and are deflected towards thereflective surface 64 inclined to the optical axis 11. The opticaloutput of a Lambertian micro-LED will now be described further.

FIG. 4C is a schematic graph illustrating in at least one catadioptriccross-sectional plane the profile 504 of simulated output luminousintensity 500 with illumination angle 502 from a refractive light inputstructure 66 onto a nominal detector plane 17 arranged between the lightinput structure 66 and the reflective surface 64 and arranged to receivelight before incidence onto the reflective surface 64. Features of thearrangement of FIG. 4C not discussed in further detail may be assumed tocorrespond to the features with equivalent reference numerals asdiscussed above, including any potential variations in the features.

Profile 504 has a dip 505 in directions that are on-axis and thusreduced luminous intensity is directed towards the axial location of thecusp 69 c of the refractive light input structure 68 of FIG. 4A. Facets67 a, 67 b provide increased luminous intensity in directions near peak507 that illuminate reflective facets 74 of the rear reflective surfacein FIG. 4, and increase luminous intensity of light rays 308 that aredirected to the refractive light output structure 56.

Advantageously increased luminous intensity is provided in the region ofthe refractive light output structure 56 and the uniformity of outputacross the transmissive output surface 52 may be increased. Further thethickness of the LED support substrate 50 and the total thickness 75 maybe reduced. Light may be provided with a spatial and angular luminousintensity distribution that matches other regions of the output surface52. The uniformity of the display from a wide range of viewing anglesmay be maintained, minimising image mura.

In the at least one catadioptric cross-sectional plane the distance 75between the transmissive output surface 52 and reflective surface 64 isless than 750 micrometres, preferably less than 500 micrometres and morepreferably less than 250 micrometres. Such low thickness can be achievedby (i) light guiding within the catadioptric optical array (ii) lowthickness of the output microstructure 56 and (iii) use of reflectiveoptics and (iv) the low thickness of the reflective substrate providedby refractive input microstructure 66. Advantageously a thin andflexible LCD display may be provided with high dynamic range localdimming operation.

The output directional distribution of an illustrative embodiment willnow be described.

FIG. 5A is a schematic graph illustrating in one catadioptriccross-sectional plane the directional distribution 520 from thecatadioptric array 100 of the present embodiments in comparison to aLambertian directional distribution 530; and FIG. 5B is a schematicgraph illustrating in one catadioptric cross-sectional plane the solidangle of a normalised directional distribution 520 in comparison to anormalised Lambertian directional distribution 530.

Luminous intensity is a measure of the energy density in a light coneand is the number of lumens per unit solid angle. In the presentembodiments the luminous intensity half maximum solid angle describesthe subtended size of the illumination output cone for which theluminous intensity is half of the peak luminous intensity in eachdirection.

Luminance of a display is determined by the luminous intensity persubtended unit area. A Lambertian surface has a has a luminance that isindependent of viewing angle and thus luminous intensity that isproportional to the cosine of the angle of observation to the normaldirection to the surface.

The luminous intensity half maximum solid angle is the solid angledefined by the cone of light in which the luminous intensity in anydirection falls to 50% of the peak luminous intensity. The solid angle Ωof a symmetric cone of full width half maximum angle 2θ is given byEquation 1.Ω=2π*(1−cos θ)  Equation 1

A Lambertian light source has a cosine distribution of luminousintensity such that the FWHM 542 illustrated in FIG. 5B is 120 degreesand the half angle, θ is 60 degrees. In the two-dimensional arrays ofthe present embodiments described in FIG. 3, the directionaldistribution is also two dimensional, so that the profiles 520, 530 arerepresentative of the solid angle of the output.

In the present embodiments, the output is directional, that is the lightoutput distribution 540 thus has a luminous intensity half maximum solidangle that is smaller than the luminous intensity half maximum solidangle of the light output distribution from each of the plurality ofmicro-LEDs 3 (that have substantially Lambertian output). The presentembodiments achieve half maximum solid angles that are less than πsteradian and the half cone angle θ in a single cross-sectional plane isless than 60 degrees, preferably less than approximately 40 degrees,more preferably less than approximately 30 degrees and most preferablyless than approximately 20 degrees. In other words, the ratio ofluminous intensity half maximum solid angle of the present embodimentsto the luminous intensity half maximum solid angle of a Lambertian lightsource is less than 1, preferably less than 50% and more preferably lessthan 25%. For a privacy display the ratio is most preferably less than10%.

In the present disclosure, the angular directional distribution refersto the distribution of luminous intensity for a point on the display, inother words the angular directional distribution is the spread of raydensity with angle for the point. The uniformity of a display representsthe spatial distribution across the catadioptric optical array 100 forany given viewing angle.

The simulated optical output of the illustrative embodiment of FIG. 4Awill now be described.

FIG. 5C is a schematic graph illustrating the simulated variation 519 byraytracing of total output luminous intensity 500 against output angles502 for the arrangement of FIG. 4A. The smoothed variation 520illustrates the profile shape for a higher ray count.

FIG. 5C illustrates the integrated luminous intensity 500 at each angleof output for all positions 502 across the output aperture of thecatadioptric optical element 38. A FWHM 540 (full angular width for halfmaximum luminous intensity) of less than 50 degrees may be achieved andlow output luminance at angles greater than 40 degrees from the surfacenormal direction. In the rotationally symmetric illumination system ofFIG. 4A, the luminous intensity half maximum solid angle Ω is determinedfrom the FWHM 540 that is 48 degrees, thus ratio of luminous intensityhalf maximum solid angle to the luminous intensity half maximum solidangle of a Lambertian light source is 17%.

The FWHM 540 with cross sectional cone half angle θ of 25 degreesillustrated in FIG. 5C achieves a luminous intensity half maximum solidangle of 0.19π, and is thus substantially less than the luminousintensity half maximum solid angle of a Lambertian diffuser.

Advantageously for the same power consumption, increased head-onluminance may be provided in comparison to the output directly from theMicro-LEDs 3. Display brightness and efficiency is increased incomparison to Lambertian emission.

The variation of luminous intensity with distance from the optical axis11 will now be described.

FIG. 6 is a schematic graph illustrating the simulated variation 521 byraytracing of total output luminous intensity against position acrossthe output aperture 502 for the arrangement of FIG. 4A. FIG. 6illustrates the variation 521 of integrated luminous intensity for allangles at each position 502 across the output aperture of the respectivecatadioptric optical element 38.

The variation in luminous intensity 500 with distance 508 from theoptical axis 11 is determined by the reflective and refractive structuredesigns including the locations and angles of the input structures 66,68facets 70, 72, 74 and planar regions 71 on the reflective surface 64 andrefractive light output structure 56. To provide increased spatialuniformity across the array, the arrangement of at least facets 70, 7274 may be modified and further diffusers may be provided on the outputof the catadioptric optical array 100.

Desirably the variation 521 increases in luminous intensity proportionalto the distance from the optical axis 11 as illustrate by profile 522.Such an increase in luminous intensity provides compensation for theincrease in the circumference or length of the light extracting facets70, 72, 74 with the distance from the optical axis, and thus maintains auniform luminous intensity per unit area, achieving uniform luminance

Advantageously uniform output luminance may be provided for a wide rangeof viewing directions in a rotationally symmetric catadioptric opticalelement 38.

It would be desirable to reduce the number of alignment steps duringmanufacture of the catadioptric optical element array 100.

FIG. 7 is a schematic diagram illustrating in top view light rays from amicro-LED 3 and catadioptric optical element 38 comprising a planartransmissive surface 62 for a catadioptric input substrate 60. Thus FIG.7 illustrates a planar input surface for the transmissive input surface62 of the input substrate 60 in comparison to the microstructured inputstructure 66 of FIG. 4A. Features of the arrangement of FIG. 7 notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

Advantageously during fabrication of input substrate 60, an alignmentstep to align input microstructure 66 with reflective input structure 68is not provided, reducing complexity and cost.

The luminous intensity profile at plane 17 for the planar input surface62 will now be described.

FIG. 8 is a schematic graph illustrating in at least one catadioptriccross-sectional plane the simulated profile 506 of output luminousintensity 500 with illumination angle 502 from a planar transmissiveinput surface 62 onto the detector plane 17 by way of comparison withFIG. 4C. In comparison to the profile 504 in FIG. 4C, profile 506 has nodip for light rays that are substantially parallel to the optical axis11, so that less light is directed to refractive light output structure56 by light rays 308 as illustrated in FIG. 4A. The density ofmicrostructures 74 may be increased to compensate for the reduction inoutput luminance.

The simulated optical output of the structure similar to FIG. 7 will nowbe described.

FIG. 9A is a schematic graph illustrating the variation of total outputluminance against output angles for the arrangement of FIG. 7; and FIG.9B is a schematic graph illustrating the variation of total outputluminance against position across the output aperture for thearrangement of FIG. 7. In comparison to FIGS. 5A-C, increased luminanceis seen at angles near to 45 degrees, there is increased non-uniformitywith viewing position and less light is directed from the region of theoutput structure 56.

In the present embodiments it would be desirable to diffuse the outputfrom the catadioptric optical array 100 to provide increased spatial andangular uniformity. Returning to the description of FIG. 7, furthersurface relief diffuser structures 352 a, 352 b are arranged on at leastone surface 52, 54 of the transmissive LED support substrate 50. Infabrication, surface relief diffusers 352 may be formed with the sametool that form structures 56 to reduce cost. Further the substrate 50may have some bulk diffusion property, for example provided by fillermaterial 354.

Advantageously mura effects arising from visibility of facets 70, 72, 74may be reduced. Further, light scatter may be provided for polarisationrecirculation, increasing efficiency.

It would be desirable to provide light cone angular output that issubstantially the same in the at least one catadioptric cross-sectionand in the direction orthogonal to the at least one catadioptric crosssection, such that the output cone angles are uniform across thecatadioptric optical array 100.

FIG. 10 is a schematic diagram illustrating in front view an arrangementof diffuser provided on at least one of the surfaces 52, 54 of thetransmissive LED support substrate 50. Features of the arrangement ofFIG. 10 not discussed in further detail may be assumed to correspond tothe features with equivalent reference numerals as discussed above,including any potential variations in the features.

Diffuser structures 352 may comprise radially extended lens surfacesthat provide diffusion in a direction that is orthogonal to the at leastone catadioptric optical cross section illustrated in FIG. 7. Inoperation, light from facets 70 may have an angle of illumination asillustrated by profile 520 in FIG. 5C or FIG. 9A. In the planeorthogonal to the cross-sectional plane, the spread of light may berelated to the size of the micro-LED 3. Such angle may vary withdistance from the micro-LED 3 and may be different from the angle of theprofiles 520. It would be desirable to achieve a light cone angleorthogonal to the cross-sectional plane that is similar to the coneangle 340 in the cross-sectional plane.

The radial lenses of FIG. 10 may be provided with a constant radius ofcurvature and thus the sag of the lenses increases with distance fromthe optical axis 11. Diffusion may be increased by the higher sag forlight that is output further from the micro-LED 3 and advantageouslyincreased uniformity of light cone in a plane orthogonal to the at leastone cross-sectional plane of FIG. 7. Advantageously improved spatial andangular uniformity of directional distribution may be achieved.

It would be desirable to provide displays with high spatial and angularuniformity, very low bezel widths and with free form shapes. Thearrangement of the optical structures of the present embodiments willnow be considered further in front view.

FIG. 11A is a schematic diagram illustrating in front view arrangementsof light reflecting facets 70,72 of the reflective surface 64 with ahexagonal extent 90 and hexagonal packing of catadioptric opticalelements 38. Features of the arrangement of FIG. 11A not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

In the plane of the catadioptric array 100 the light reflecting facets70, 72, 74 are circularly symmetric. The plurality of light reflectingfacets 70 of a catadioptric optical element 38 are concentric with theoptical axis 11 of said catadioptric optical element 38.

The propagation of light rays 300, 304, 306, 308 as described in FIG. 4Aare illustrated. Light from micro-LED 3 aligned to the optical axis 11is directed through each of the pixels 220, 222, 224 of the spatiallight modulator 200 with substantially the same luminous intensitydirectional distribution. Advantageously high spatial uniformity may beprovided with high efficiency.

In other embodiments (not illustrated), the light reflecting facets maybe elliptically symmetric about the optical axis 11. Elliptical lightextracting facets 70, 72, 74 may provide asymmetric light output conesin orthogonal directions, for example to provide preferential viewingcomfort in one direction compared to the other. For example, a fixedlandscape display may have higher viewing freedom in the lateraldirection compared to the elevation direction. Advantageously increasedefficiency or increased viewing freedom may be provided.

Referring to the lower edge of the display of FIG. 2, in seal region 209no pixels are provided and outside the seal region 209 the liquidcrystal layer 208 comprises pixels 220, 222, 224. The width of the sealregion may be 1 mm or less. FIG. 11A further illustrates raypath 301that may be reflected from reflective material 61 arranged on the edgesof the catadioptric optical array 100 as illustrated by ray 301 in FIG.2. Advantageously, display bezel width may be minimised and free formdisplay shapes achieved such as the curved display corner illustrated inFIG. 11A.

It would be desirable to provide further control of display luminanceuniformity.

In a rotationally symmetric catadioptric optical element 38 the luminousintensity of extracted light falls with distance from the optical axis11 as the total area of the reflecting facet 70 increases with distance,being proportional to the circumference of the facet. The length ofcircular reflecting facets 70 increases in proportion to the radius. Itis desirable to maintain a uniform luminance across the area of thecatadioptric optical array 100.

Returning to FIG. 6, a cross sectional variation of luminous intensity522 to is illustrated achieve a uniform luminance in a rotationallysymmetric catadioptric optical element 38. The provision of uniformextracted luminance from the catadioptric optical element will now bedescribed further.

The extracted luminance over the area of a catadioptric optical element38 is determined by the incident luminous intensity in any notionalregion across the element 38 and the area of extraction facets 70 insaid area. For facets 70 that are arranged with equal width and equalpitch, the total facet 70 area is determined by the facet 70circumference and increases proportionally with distance from themicro-LED 3. For a fixed luminous intensity in each notional region, theoutput luminance will fall towards the edge of the element, and create anon-uniformity. It would be desirable to maintain uniform luminanceacross the area of the element 38 by increasing the luminous intensityof extracted light from the centre to the edges of the element 38. Tocontinue the illustrative example, a desirable increase of luminousintensity towards the edges is illustrated in FIG. 6 and FIG. 9B.

In the present embodiments, as illustrated in FIG. 4A, some increasedluminous intensity at the outer notional regions of the element 38 isachieved by guiding light from the micro-LED 3 to the outer regions.

Further, in the embodiment of FIG. 11A, the light reflecting facets 70of each catadioptric optical element 38 are arranged with a separationthat decreases with distance from the optical axis 11 of thecatadioptric element. Thus, the number of facets 70 per unit areaincreases at high radius, and such an increase in facet 70 densityprovides increased light extraction that compensates for the increasedfacet 70 circumference.

Further light ray 312 is shown for light rays that are reflected fromthe edge reflector 61. Advantageously very low bezel widths may beachieved with free-form shapes.

Further arrangements to achieve uniform spatial uniformity of luminancewill now be described.

FIG. 11B is a schematic diagram illustrating in front view arrangementsof light reflecting facets 70 of the reflective surface 64 with a squareextent 90 and a square packing of catadioptric optical elements 38.Square extent 90 may provide a different mura visibility to thehexagonal extent of FIG. 11A. Features of the arrangement of FIG. 11Bnot discussed in further detail may be assumed to correspond to thefeatures with equivalent reference numerals as discussed above,including any potential variations in the features.

In comparison with the arrangement of FIG. 11A, the spacing of thefacets 70, 72, 74 in at least one catadioptric cross sectional plane maybe similar. Additional planar regions 77 are provided that reduce thetotal length of each facet 70, 72, 74, with length that varies dependingon facet function and distance from the optical axis 11. Thus facets 70,72, 74 may comprise facet segments 79, the length of the facet segmentsincreasing with distance from the micro-LED 3. Output diffusers, forexample arranged on surfaces 52, 54 of transmissive LED supportsubstrate 50 may be arranged to provide uniform output in gap betweenfacet segments. Thus in the plane of a catadioptric optical element 38the length of the light reflecting facets 70 increases with distancefrom the optical axis 11 of the respective catadioptric optical element38. Typically, the facets 70 will have an area in the plane of thecatadioptric optical element such that the total area of the lightreflecting facets 70 increases with the distance from the optical axis11 of the respective catadioptric optical element 38. Further, the totalarea of the light reflecting facets 70 is proportional to the distancefrom the optical axis 11 of the respective catadioptric optical element38. Compensation for the non-linear variation in luminous intensity ofFIG. 9B may be provided.

FIG. 11C is a schematic diagram illustrating in perspective side view aone dimensional catadioptric optical element array with reflectivesurface structure of the type illustrated in FIG. 2 and alignedmicro-LED array. Features of the arrangement of FIG. 11C not discussedin further detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

In comparison to the arrangement of FIG. 1, the catadioptric opticalelement array 60 is extended in the y-direction. Such an arrangementprovides control of cone angle in the x-z plane and substantiallyLambertian output in the y-z plane. The display may be observed withsubstantially the same luminance for rotation about the x-axis and withreduced luminance for rotation about the y axis. Advantageouslycomfortable viewing freedom may be achieved for a head on user forvarious display orientations. Further, manufacture of such componentsmay also be conveniently achieved.

FIG. 11D is a schematic diagram illustrating in side view a reflectivesurface structure for a catadioptric optical element array; FIG. 11E isa schematic diagram illustrating in side view a catadioptric opticalelement array comprising the structure of FIG. 11D; and FIG. 11F is aschematic diagram illustrating in perspective side view a twodimensional catadioptric optical element array with reflective surfacestructure of the type illustrated in FIGS. 11D-E and aligned micro-LEDarray. Features of the arrangements of FIGS. 11D-F not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

In comparison to the arrangement of FIG. 1 for example, the facets 64,65 may have different tilt angles and the outer surface 70 may belinear. Such a structure may be tooled with reduce cost and complexity.

It may be desirable to achieve at least two different luminance angulardistributions, for example to achieve switching between a wide angle andprivacy mode of operation.

FIG. 12A is a schematic diagram illustrating in front view a reflectivesurface structure for a catadioptric optical element array and first andsecond micro-LED arrays arranged to provide switching between a narrowangle and wide angle field of view. Features of the arrangement of FIG.12A not discussed in further detail may be assumed to correspond to thefeatures with equivalent reference numerals as discussed above,including any potential variations in the features.

The illumination apparatus comprises first plurality of LEDs 3A andfurther comprises a second plurality of LEDs 3B arranged in an LEDarray, wherein the second plurality of LEDs 3B are micro-LEDs ormini-LEDs. Each optical axis 11 is offset from one or more of the LEDs3B of the second plurality of LEDs, and each of the LEDs 3B of thesecond plurality of LEDs is offset from the optical axis 11 of at leastone of the catadioptric optical elements (e.g. at a distance 711 fromthe optical axis 11). Each optical axis 11 is aligned in correspondencewith an LED 3A of the first plurality of LEDs, and each of the firstplurality LEDs 3A is aligned in correspondence with the optical axis 11of one of the catadioptric optical elements. In the embodiment of FIG.12A the first LEDs 3A are arranged at the optical axis 11 (such that thedistance 711 is zero) and the second LEDs 3B are arranged at some of theapices of the hexagonal catadioptric optical elements 38.

Drive controller 130A is arranged to provide LEDs 3A with drive signalsthat may comprise image data to achieve high image contrast by means oflocal area dimming. Drive controller 130B is arranged to provide LEDs 3Bwith drive signals that may comprise image data to achieve high imagecontrast by means of local area dimming.

FIG. 12B is a schematic graph illustrating the polar variation ofluminous intensity for one region on the backlight for the firstmicro-LED array of FIG. 12A; and FIG. 12C is a schematic graphillustrating the spatial uniformity of luminous intensity for the firstmicro-LED array of FIG. 12A for a normal direction and for anillustrative embodiment with a micro-LED 3 pitch of 2 mm in the x-axis.

Advantageously a narrow cone angle can be achieved with relatively highspatial uniformity. Addition of a diffuser may be used to increasespatial uniformity while increasing solid angle of the output lightcone. Desirably after diffusion, the FWHM of the output light cone isless than 30 degrees, preferably less than 25 degrees and mostpreferably less than 20 degrees.

FIG. 12D is a schematic graph illustrating the polar variation ofluminous intensity for one region on the backlight for the secondmicro-LED array of FIG. 12A; and FIG. 12E is a schematic graphillustrating the spatial uniformity of luminous intensity across thearray for the second micro-LED array of FIG. 12A for a pitch ofcatadioptric optical elements of 2 mm.

As shown in FIGS. 12D-E, the light output distribution of there-directed light provided by each catadioptric optical element usinglight output from the second plurality of LEDs 3B has a luminousintensity half maximum solid angle that is greater than the luminousintensity half maximum solid angle of the of the re-directed lightprovided by each catadioptric optical element using light output fromthe first plurality of LEDs 3A that is shown in FIG. 12B. Advantageouslythe light may be spread over a wide field of view by driving the LEDs3B.

Advantageously a display may be provided that can switch between wideangle mode for use by multiple users and for wide range of viewingdirections; and a narrow angle mode of operation that may provideprivacy viewing, low stray light operation and high power efficiencywith extended battery lifetime. Further very high luminance may beachieved in on-axis directions for low power consumption.

The distance 711 of the micro-LEDs 3A from the optical axis 11 mayfurther be modified across the area of the illumination apparatus, suchthat the direction of peak luminance is pointed at a nominal observerlocation. The output may be pupillated such that for an observer in anominal viewing location advantageously display luminance uniformity maybe increased.

Alternative arrangements for reflective light input structure 68 andrefractive light output structure 56 will now be described.

FIG. 13A is a schematic diagram illustrating in top view light the inputregion of a catadioptric optical element 38 and aligned micro-LED 3 inat least one catadioptric cross-sectional plane through its optical axis11. Features of the arrangement of FIG. 13A not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

The operation of the refractive light output structure 56 arranged onthe transmissive output surface 52 will now be described. In theembodiment of FIG. 13A, the refractive light output structure 56comprises a concave refractive surface 55 arranged to provide negativeoptical power. Light rays 308 from reflective facets 74 are redirectedby concave surface 55 to reduce the angle of the rays to the opticalaxis 11, thus improving the collimation of the output in the region thatis otherwise shadowed by the opaque mask 7. Features of the arrangementof FIG. 13A not discussed in further detail may be assumed to correspondto the features with equivalent reference numerals as discussed above,including any potential variations in the features.

FIG. 13A further illustrates reflective light input structure 68 maycomprise linear inner surfaces 69 a, 69 b. Advantageously the surfaces69 a, 69 b may be more conveniently tooled than the curved innersurfaces of FIG. 4A.

It would be desirable to provide a backlight with increased robustnessand reduced sensitivity to thermal variations.

FIG. 13B is a schematic diagram illustrating in top view light rays froma micro-LED and catadioptric optical element 38 comprising a LED supportsubstrate that is attached to the catadioptric substrate. Transparentmaterial 59 is provided between the first surface 54 of the transmissiveLED support substrate 50 and the transmissive input surface 62. Featuresof the arrangement of FIG. 13B not discussed in further detail may beassumed to correspond to the features with equivalent reference numeralsas discussed above, including any potential variations in the features.

Light rays from the plurality of micro-LEDs 3 is guided within thecatadioptric optical array 100 between the reflective surface 64 and thesecond surface 52 of the transmissive LED support substrate 50.Advantageously such a backlight may achieve increased robustness tothermal variations and mechanical deformations.

FIG. 13B further illustrates an embodiment wherein a wavelengthconversion layer 205 is arranged to receive light from catadioptricoptical array 100. The light rays 300, 304, 306, 308 that propagatewithin the catadioptric optical array 100 may comprise blue light forexample and may be incident on the separate wavelength conversion layer205 wherein some of the light is converted to yellow light and whitelight is output from the wavelength conversion layer onto a diffuser360.

Alternatively, the micro-LED may be provide ultra-violet light and thewavelength conversion layer 205 may be provided to achieve white outputlight.

The operating temperature of the wavelength conversion layer 205 may bereduced in comparison to the conversion layer 5 aligned to the micro-LED3 in FIG. 4A and advantageously efficiency of colour conversion may beincreased. Further the wavelength conversion layer 205 may comprisequantum dot materials that may encapsulated within appropriateprotective substrates to inhibit the conduction of water and/or oxygen.Alternatively, the wavelength conversion layer 205 may be provided bythe transmissive LED support substrate, wherein wavelength conversionmaterial may be provided on the first surface 54, within the bulk of thesubstrate 50 or on the output surface 52. Such wavelength conversionlayer 205 also achieves diffused output light, providing increaseduniformity.

FIG. 13B further illustrates part of a circuit 720 controlling one ormore micro-LEDs 3 and located within the LED array. Circuit 720 maycomprise one or more integrated circuits and may comprise one or moreTFTs and may comprise one or more passive components such as capacitors.Advantageously integrated circuit elements may be provided across thearray rather than at the edges, achieving reduced bezel width andfree-form shapes.

FIG. 13C is a schematic diagram illustrating in top view light rays froma micro-LED 3 and catadioptric optical element 38 comprising atransparent LED support substrate wherein the second side of thetransparent substrate is attached to the catadioptric substrate.Features of the arrangement of FIG. 13C not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

In comparison to FIG. 13B, the LED 3 is arranged to illuminate throughtransparent LED support substrate 50 which is bonded to the reflectiveinput structure 60. Advantageously device thickness may be reduced whileachieving guiding rays 300 to increase lateral uniformity.

It would be desirable to provide a flexible backlight.

FIG. 14A is a schematic diagram illustrating in top view a catadioptricoptical array and plurality of micro-LEDs wherein the reflective inputstructures 60 do not comprise an integrated body. Features of thearrangement of FIG. 14A not discussed in further detail may be assumedto correspond to the features with equivalent reference numerals asdiscussed above, including any potential variations in the features.

Each catadioptric optical element 38 a, 38 b is separated from adjacentelement 38 b by gap 97 that is arranged to provide some mechanicaldeformation region during flexing of the substrates. Reflective coating65 is arranged to extend over the outer surface of each element 38including the reflective sides 36 of each catadioptric optical element38. In operation, light that is guided within the catadioptric opticalarray is reflected from the sides 36. Advantageously increaseddeformation of the catadioptric optical elements may be provided toenable the catadioptric optical array to confirm to a curved shape in atleast one dimension. Features of the arrangement of FIG. 14A notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

Further in the embodiment of FIG. 14A, the catadioptric optical array100 is illustrated as comprising input substrate 60 formed on thetransparent LED support substrate 50. Advantageously robustness ofalignment may be increased.

FIG. 14B is a schematic diagram illustrating in top view a curvedbacklight for a curved spatial light modulator 200 such as an LCDcomprising flexible substrates. Features of the arrangement of FIG. 14Bnot discussed in further detail may be assumed to correspond to thefeatures with equivalent reference numerals as discussed above,including any potential variations in the features.

Light rays 300, 302 that are output from the curved display may have anincreased cone angle in comparison to that illustrated in FIG. 5C forexample, such that the roll off of display brightness across the curveddisplay for an observer in a fixed viewing position is reduced.Increased diffusion may be provided by diffuser 360 and diffusingelements in and on substrate 50. Features of the arrangement of FIG. 14Bnot discussed in further detail may be assumed to correspond to thefeatures with equivalent reference numerals as discussed above,including any potential variations in the features.

It would be desirable to address an array of micro-LEDs 3 in anefficient way. It would also be desirable to address micro-LEDs 3 with areduced number of column electrodes 700 and row electrodes 702.

It may be desirable to reduce the complexity of the reflective surface70.

FIG. 15A is a schematic diagram illustrating in side perspective view adisplay apparatus comprising a backlight comprising a micro-LED 3 and acatadioptric optical element 38 arranged to illuminate an LCD 200wherein the reflective surface 70 of the catadioptric optical element 38does not comprise regions 71 arranged to provide guiding with thecatadioptric optical element 38; FIG. 15B is a schematic diagramillustrating in side view the catadioptric optical element 38 of FIG.15A and the micro-LEDs 3 are arranged in alignment with the optical axes11 of the respective aligned catadioptric optical elements 38; and FIG.15C is a schematic diagram illustrating in side perspective view a twodimensional catadioptric optical element and aligned array ofmicro-LEDs.

The surface 70 is arranged to provide output rays 370 with a narrow coneangle. In comparison to the arrangement of FIG. 2 or FIGS. 11D-E, thereflective surface 70 has a simpler non-faceted shape so thatadvantageously light scatter and tooling complexity and cost may bereduced. Further, the aperture width 715 may be arranged to match theluminance output profile of the light injected at the transmissive inputsurface 62, that is defined by the critical angle θ_(c) in the case ofthe input refractive surface being planar. As illustrated in FIG. 8, theluminous intensity roll-off at the edge of the reflective aperture is70% of the peak luminous intensity and desirably a relatively spatiallyuniform output may be achieved that can be further corrected by means ofdiffusion in the backlight apparatus including on diffusers attached tothe LCD as illustrated elsewhere herein.

FIG. 15D is a schematic diagram illustrating in side perspective view aone dimensional catadioptric optical element and aligned array ofmicro-LEDs. In comparison to the arrangement of FIG. 15C, a onedimensional luminance roll-off may be achieved, advantageouslyincreasing uniformity and viewing freedom for display rotations aboutthe x-axis. Further the complexity of tooling may be further reduced.

It may be desirable to provide off-axis viewing of a display operatingin privacy mode or with low stray light.

FIG. 15E is a schematic diagram illustrating in side view a catadioptricoptical element arranged wherein the micro-LEDs 3 are arranged offset bydistance 717 from the optical axes 11 of respective aligned catadioptricoptical element 38. In comparison to the arrangement of FIG. 15B,off-axis illumination of the LCD 200 may be provided. Advantageously adisplay that is viewable from a limited range of off-axis angles. Such adisplay may for example provide a centre console display in a vehiclethat is only visible for a driver or passenger. Further second pluralityof LEDs may be provided (not shown) so that such a display is switchedbetween wide angle and narrow angle modes.

It would be desirable to provide a display that is visible from morethan one direction

FIG. 15F is a schematic diagram illustrating in side view a catadioptricoptical element arranged wherein the catadioptric optical elementwherein first and second micro-LEDs are arranged offset from the opticalaxes of respective aligned catadioptric optical element by first andsecond distances respectively.

First and second pluralities of LEDs 3A, 3B may be provided where eachLED is offset from the optical axis 11 of the respective alignedcatadioptric optical element 38. Output rays 374A may be provided in onedirection and output rays 374B provided in a different direction. Such adisplay may provide low stray light images for two users, for examplethe driver and passenger of a vehicle.

Further the backlight controller 130 and display controller 230 maycooperate to provide a dual view display. In a first phase of operationthe LEDs 3A are illuminated and a first image displayed on the LCD 200.In a second phase of operation the LEDs 3B are illuminated and a secondimage display on the LCD 200. The first and second images may bedifferent. Advantageously a dual view display may be provided.

FIG. 15G is a schematic diagram illustrating in side view a catadioptricoptical element 38 similar to FIG. 15B wherein the reflective surface 70is provided by a Fresnel reflector. Advantageously thickness is reduced.

Features of the arrangement of FIGS. 15A-G not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

FIG. 16A is a schematic diagram illustrating an addressing system forthe plurality of LEDs. The electrodes 7, 8 of FIG. 1 of each of themicro-LEDs 3 of the plurality micro-LEDs 3 are respectively connected toone column addressing electrode 700 and one row addressing electrode 702to form a matrix. In this embodiment an array of current sources 716 isused to drive the addressing electrodes 700. The voltage on each of therow electrodes 702 is pulsed in sequence to scan or address the array ofmicro-LEDs 3. A current source 716 may be provided for each columnelectrode 700 or may be time multiplexed (shared) amongst a set ofcolumn electrodes 700. The micro-LEDs 3 have a relatively sharp voltagevs. current curve and can be operated with very short pulses withoutcross-talk between them. The array of micro-LEDs 3 forms an addressablebacklight or a display without the need for additional active componentssuch as TFTs or integrated circuits at each pixel. However, all theenergy to illuminate the micro-LEDs must be provided during theaddressing pulse. Advantageously the addressing matrix is simple and lowcost.

It would be desirable to reduce the peak LED current while maintaininglight output levels.

FIG. 16B is a schematic diagram illustrating another addressingembodiment for the plurality of LEDs. The micro-LEDs 3 of the pluralitymicro-LEDs 3 are addressed by column addressing electrodes 700 and rowaddressing electrodes 702 to form a one-dimensional or two-dimensionalmatrix. For clarity only one micro-LED 3 and one column electrode 700and one row electrode 702 of the matrix is shown. FIG. 16B differs fromFIG. 16A in that each micro-LED 3 has associated with it an integratedcircuit 708 which includes a storage or memory or latching function. Theintegrated circuit 708 may be an analog or digital circuit and may beembodied as a separate chip located using a method that is similar tothe micro-LED 3 location method or may be embodied with TFTs. Theintegrated circuit 708 may be provided with one or more additionalsupply potentials V1, V2 (only V1 shown). The drive circuit 720 shown inFIG. 13B is comprises integrated circuit 708. When the row electrode 702is pulsed the clock input 710 of integrated circuit 708 stores thecolumn electrode 700 voltage connected to the Data input 712. The output714 of the integrated circuit 708 drives the micro-LED 3. The other endof the micro-LED is connected to supply potential V3. The integratedcircuit 708 may include a voltage to current converter. The potential V3and the anode and cathode connections of the micro-LED 3 may beconfigured so that the micro-LED is forward biased and emits light. Theintegrated circuit 708 provides drive to the micro-LED 3 for longer thanthe duration of the addressing pulse on row electrode 702 and the peakcurrent drive to the micro-LEDs 3 is reduced. Advantageously the peakcurrent in each micro-LED 3 is reduced.

FIG. 16C is a schematic diagram illustrating another addressingembodiment for the plurality of LEDs. The micro-LEDs 3 of the pluralitymicro-LEDs 3 are addressed by column addressing electrodes 700 and rowaddressing electrodes 702 to form a 1-dimensional or 2-dimensionalmatrix or array. Drive circuit 720 illustrated FIG. 13B comprises TFT706, amplifier 704 and capacitor 718. In this embodiment row electrodes702 is connected to the gate of TFT 706 and when the row addressingelectrode 702 is pulsed, the data from column addressing electrode 700is stored on capacitor 718. Capacitor 718 may be small compared to thattypically used in a matrix to drive an LCD panel and may be provided bythe input capacitance of amplifier 704. The amplifier 704 may drive oneor more micro-LEDs 3. Amplifier 704 may be provided with 1 or moresupply voltages (not shown). Amplifier 704 may include a voltage tocurrent converter circuit. Amplifier 704 may drive one or more stringsof one or more micro-LEDs 3. In this example embodiment two strings of 3micro-LEDs 3 are illustrated. The other end of the strings of micro-LEDs3 is connected to potential V2, and the voltage output from amplifier704 must be greater than voltage V2 by the combined forward voltage drop(Vf) of the string of micro-LEDs 3 in order for the micro-LEDs 3 toilluminate.

It would be desirable to provide some resilience of the display orbacklight to failure of individual micro-LEDs 3. The failure may be anopen circuit which may be caused for example by mis-placement ofmicro-LEDs 3 in manufacture or may be a short circuit for example fromdamaged electrode wiring.

FIG. 16D is a schematic diagram illustrating another addressingembodiment for the plurality of LEDs. The micro-LEDs 3 of the pluralitymicro-LEDs 3 are addressed by column addressing electrodes 700 and rowaddressing electrodes 702 in a one-dimensional or two-dimensionalmatrix. In this embodiment the micro-LEDs 3 are arranged in bridgedstrings. This configuration provides some immunity to individualmicro-LED 3 being open circuit or short circuit. Advantageously thedisplay or backlight can be fault tolerant and more reliable.

Features of the arrangements of FIGS. 16A-D not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

It would be desirable to provide a large size display with precise anduniform alignment of micro-LEDs 3 to the optical axes of catadioptricoptical elements 38 to achieve uniform output spatial and directionalluminous intensity directional distribution. A method to form anillumination apparatus will now be further described.

A shaped tool 600 may be provided as shown in FIG. 17A which is aschematic diagram illustrating in side perspective view a tool 600 withfeatures 666 that have separation s2 in at least a first direction forforming a plurality of refractive light input structures 66.

In a first step an input substrate 602 is provided as shown in FIG. 17Bwhich is a schematic diagram illustrating in side perspective view aninput substrate comprising a plurality of refractive light inputstructures. In the second step the tool 600 may be arranged to providethe refractive light input structures 66 on a side of a transparentsubstrate 602 in a curable layer 604 of transparent material, such as aUV cast acrylate material. The structures 66 may be provided by otherknown replication methods such as injection moulding or hot embossing.The separation of the features s3 in the at least first direction may bethe same as the separation s2, or may have a calibrated adjustment totake into account differences in thermal expansion of the materials ofthe tool and substrate 602 during replication.

A shaped tool 606 may be provided as shown in FIG. 17C which is aschematic diagram illustrating in side perspective view a tool 606 forforming a plurality of reflective light input structures 68, 70, 71, 72,74 with separation s4 in the at least first direction.

In a second step a reflective surface 64 is provided as shown in FIG.17D which is a schematic diagram illustrating in side perspective viewan input substrate comprising a plurality of refractive light inputstructures 66 and a reflective surface 64 comprising a pluralityreflective structures 68, 70, 71, 72, 74. The reflective surface may beprovided by UV casting onto layer 608. Alternatively, the first andsecond steps may be combined in a single process, in which the tools600, 606 are aligned prior to the replication process. The plurality ofreflective structures each arranged as an array with a separation s5 inthe at least first direction and are aligned to the input refractivestructures 66. The separation s5 is arranged to be the same asseparation s3.

In a third step a reflective coating 65 is provided as shown in FIG. 17Ewhich is a schematic diagram illustrating in side perspective view acoated input substrate 60.

Features of the arrangements of FIGS. 17A-E not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

In a fourth step a refractive light output structure 56 is provided on asubstrate 610 as shown in FIG. 18A which is a schematic diagramillustrating in side perspective view part of an LED support substrate50 comprising a plurality of refractive light output structures 56 and adiffusing structure 352 formed in layer 612 on substrate 610 for exampleby UV casting. Alternatively, the structures 56, 352 may be formed bymoulding and are formed in the same material as the substrate 610.Separation s6 is arranged to be the same as separation s5.

In a fifth step an addressing electrode array may be provided as shownin FIG. 18B which is a schematic diagram illustrating in sideperspective view an LED support substrate 50 further comprising aplurality of addressing electrodes 8. Electrodes 8 may be formed bylithography, mask deposition, printing or other known methods withseparation s7 in the at least first direction that is substantially thesame as the separation s5.

In a sixth step, opaque mask regions 7 may be provided as shown in FIG.18C which is a schematic diagram illustrating in side perspective viewan LED support substrate further comprising a plurality of opaque maskregions 7. Mask regions 7 may be electrodes or may be dielectricmaterials that are formed between the electrodes 8 and substrate 610 andmay have separation s7 that is the same as separation s5 in the at leastfirst direction.

Features of the arrangements of FIG. 18A-C not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

In a seventh step a monolithic semiconductor wafer 2 may be provided asshown in FIG. 19A which is a schematic diagram illustrating in sideperspective view a monolithic LED wafer 2. For example, the monolithicwafer 2 may comprise multiple doped gallium nitride layers and may beformed on a substrate 4 that may be sapphire, silicon carbide or siliconfor example.

In an eighth step a non-monolithic array of micro-LEDs 3 a, 3 b may beextracted from the monolithic wafer 2 as shown in FIG. 19B which is aschematic diagram illustrating in side perspective view extraction of asparse array of micro-LEDs from a monolithic LED wafer 2 to providemicro-LEDs 3 a, 3 b with separation s1 in the at least first direction.

In a ninth step the non-monolithic array of micro-LEDs 3 a, 3 b may betransferred onto the transparent LED support substrate 50 as shown inFIG. 19C which is a schematic diagram illustrating in side perspectiveview placement of the sparse array of micro-LEDs 3 a, 3 b from amonolithic LED wafer 2 of FIG. 19A onto the LED support substrate ofFIG. 18C. Features of the arrangements of FIGS. 19A-C not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

Micro-LEDs 3 a, 3 b may be arranged on substrate 52 in alignment withelectrodes 8 and refractive light output structure 66. The LED supportsubstrate 50 may already be provided with drive circuit 720 comprisingfor example TFT 706 and/or integrated circuit 708 as described withreference to FIGS. 16A-D.

The LEDs of the plurality of LEDs are thus from a monolithic wafer 4arranged in an array with their original monolithic wafer positions andorientations relative to each other preserved; and wherein in at leastone direction, for at least one pair of the plurality of LEDs in the atleast one direction, for each respective pair there was at least onerespective LED in the monolithic wafer 4 that was positioned in themonolithic wafer 4 between the pair of LEDs in the at least onedirection and that is not positioned between them in the array of LEDs3.

In a tenth step, further layers (not shown) including addressingelectrodes, wavelength conversion layers and optical bonding layers maybe provided on the micro-LEDs 3 and the first surface of thetransmissive LED support substrate 50. Further electrodes mayalternatively or additionally be provided on the catadioptric inputsubstrate 60 as described in WO2012052722, incorporated herein in itsentirety by reference.

In an eleventh step an illumination apparatus may be provided as shownin FIG. 20 which is a schematic diagram illustrating in side perspectiveview assembly of a backlight 100 comprising an input substrate 60 andtransparent LED support substrate 50. Features of the arrangement ofFIG. 20 not discussed in further detail may be assumed to correspond tothe features with equivalent reference numerals as discussed above,including any potential variations in the features.

The substrate 50 may be aligned with the plurality of catadioptricoptical elements 38 with separations s5 to provide an illuminationapparatus, such that separation s5 may be the same as separation s1.Optical bonding such as optically clear adhesives may be used to provideattachment between the two substrates 50, 60 to advantageously provideincreased robustness of alignment. Advantageously large numbers ofelements may be formed over large areas using small numbers ofextraction steps, while preserving alignment to a respective array ofoptical elements. Alignment of micro-LEDs 3 to catadioptric opticalelements is described further in WO2010038025, incorporated herein inits entirety by reference.

Further for the present disclosure, micro-LEDs are unpackaged LED diechips, and are not packaged LEDs. Advantageously individual wire bondingto LEDs is not used and the number of pick and place processes issignificantly reduced.

The invention claimed is:
 1. An illumination apparatus comprising: aplurality of LEDs arranged in an LED array, wherein the plurality ofLEDs are micro-LEDs or mini-LEDs, each of the plurality of LEDs beingarranged to output light; and a plurality of catadioptric opticalelements arranged in a catadioptric optical array, each catadioptricoptical element comprising a reflective interface and a transmissiveinterface, wherein: for each catadioptric optical element, thereflective interface is arranged to receive light output from one ormore of the LEDs through the transmissive interface and to reflect thereceived light back through the transmissive interface, thereby toprovide re-directed light, wherein the re-directed light provided byeach catadioptric optical element has a narrower light cone than thelight output by each of the plurality of LEDs.
 2. An illuminationapparatus according to claim 1, wherein at least some of the light fromthe plurality of LEDs is guided, at least in part via total internalreflection, within the catadioptric optical array.
 3. An illuminationapparatus according to claim 1, wherein each of the plurality of LEDs isarranged on a first interface of at least one transmissive LED supportsubstrate, a transmissive output interface is provided by a secondinterface of the transmissive LED support substrate, and the secondinterface of the transmissive LED support substrate faces the firstinterface of the transmissive LED support substrate.
 4. An illuminationapparatus according to claim 3, wherein each catadioptric opticalelement comprises an optical axis, and wherein the LED support substrateis formed as an integrated body that extends between the optical axes ofthe plurality of catadioptric optical elements.
 5. An illuminationapparatus according to claim 3, wherein a transparent material isprovided between the first interface of the transmissive LED supportsubstrate and the transmissive interface of the catadioptric opticalelement; and the light from the plurality of LEDs that is guided withinthe catadioptric optical array is guided between the reflectiveinterface and the second interface of the transmissive LED supportsubstrate.
 6. An illumination apparatus according to claim 5, whereinthe transparent material is air.
 7. An illumination apparatus accordingto claim 3, wherein the reflective interface of each catadioptricoptical element is arranged on a first interface of an input substrate,a second interface of the input substrate facing the reflectiveinterface comprises a transmissive input interface, and the firstinterface of the transmissive LED support substrate faces thetransmissive input interface.
 8. An illumination apparatus according toclaim 7, wherein each catadioptric optical element comprises an opticalaxis, wherein for each catadioptric optical element the transmissiveinterface of the input substrate further comprises a refractive lightinput structure aligned in correspondence with a respective optical axisof the catadioptric optical element, and wherein each refractive lightinput structure is arranged between the transmissive input interface andthe reflective interface of the input substrate.
 9. An illuminationapparatus according to claim 8, wherein in at least one catadioptriccross-sectional plane through its optical axis the refractive lightinput structure comprises a plurality of pairs of oppositely inclinedrefractive input facets.
 10. An illumination apparatus according toclaim 7, wherein a transparent material with a lower refractive indexthan a material from which the input substrate is made is arrangedbetween the plurality of LEDs and the transmissive interfaces of thecatadioptric optical elements.
 11. An illumination apparatus accordingto claim 7, wherein at least some of the light from the plurality ofLEDs is guided, at least in part via total internal reflection, withinthe catadioptric optical array, and wherein the light from the pluralityof LEDs that is guided within the catadioptric optical array is guided,at least in part via total internal reflection, between the reflectiveinterface and the transmissive input interface.
 12. An illuminationapparatus according to claim 1, wherein each catadioptric opticalelement comprises an optical axis, wherein each optical axis is alignedin correspondence with a respective one or more of the LEDs, and whereineach of the LEDs is aligned in correspondence with the optical axis ofonly one of the catadioptric optical elements.
 13. An illuminationapparatus according to claim 12, further comprising a further pluralityof LEDs arranged in an LED array, wherein: the further plurality of LEDsare micro-LEDs or mini-LEDs, and each optical axis is offset from one ormore of the LEDs of the further plurality of LEDs, and each of the LEDsof the further plurality of LEDs is offset from the optical axis of atleast one of the catadioptric optical elements.
 14. An illuminationapparatus according to claim 13, wherein: for each catadioptric opticalelement, the reflective interface is arranged to receive light outputfrom one or more of the further plurality of LEDs through thetransmissive interface and to reflect the received light back throughthe transmissive interface, thereby to provide re-directed light, andthe re-directed light provided by each catadioptric optical elementusing light output from the further plurality of LEDs has a wider lightcone than the re-directed light provided by each catadioptric opticalelement using light output from the plurality of LEDs.
 15. Anillumination apparatus according to claim 1, wherein each catadioptricoptical element comprises an optical axis, and wherein for eachcatadioptric optical element of the catadioptric optical array, thetransmissive interface comprises at least one refractive light outputstructure arranged on the transmissive interface and aligned incorrespondence with the optical axis of the catadioptric opticalelement.
 16. An illumination apparatus according to claim 1, whereineach catadioptric optical element comprises an optical axis, and whereinthe input substrate is formed as an integrated body that extends betweenthe optical axes of the plurality of catadioptric optical elements. 17.An illumination apparatus according to claim 1, wherein the reflectiveinterface of the catadioptric optical array comprises a reflective layerformed on the reflective interface wherein the reflective layer extendsto cover the reflective interface of the catadioptric optical array. 18.An illumination apparatus according to claim 1, wherein the reflectiveinterface of each catadioptric optical element comprises a plurality oflight reflecting facets.
 19. A display apparatus comprising theillumination apparatus of claim 1 and a transmissive spatial lightmodulator arranged to receive light that has transmitted through thetransmissive LED support substrate.
 20. A backlight apparatus for aliquid crystal display comprising the illumination apparatus of claim 1.